-^^2.^0         N87 

Columbia  anitJem'tp^^P/^ 
mtljfCttpoflmgork 

College  of  ^fjpsiicianfl!  anb  S'urgeonjS 
ILibrarp 


GiJ^-fc  of 


Dr.  C.  F.  M&.cPoTva-!<i 


'">-.. . 


-A<H' 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/textbookofdentalOOnoye 


A  TEXT-BOOK 


OF 


DENTAL  HISTOLOGY 


AND 


EMBRYOLOGY 


IXCLUDIXG 


LABORATORY  DIRECTIONS 


BY 

FREDERICK  BOGUE  NOYES,  B.A.,  D.D.S. 

PROFESSOR   OF  HISTOLOGY,   NORTHWESTERN   UNIVERSITY   DENTAL   SCHOOL 


WITH    350    ILLUSTRATIONS    A^  D  19    PLATES 


LEA    &    FEBIGER 

PHILADELPHIA    AND    NEW    YORK 
1912 


C^/VcvwjMtU    Vcx^^c^cJ?c/>8c^,?) 


Entered  according  to  Act  of  Congress,  in  the  year  1912,  by 

LEA    &   FEBIGER 

in  the  Office  of  the  Librarian  of  Congress.     All  rights  reserved. 


^ 


2_ 


TLo  ms  ffatbet 

2)r«  I£^num&  IRoijee 

Mboee  long  aiiD  active  professional  career  bas  been  Devoted, 

witbout  personal  ambition  or  selfisb  advancement,  to 

tbe  0OOD  ot  tbe  Dental  profession,  anO  wbose 

unselfisbness  anD  sacrifice  bave  maDe 

possible  all  tbat  "IT  bave  C>one 

or  mas  accomplisb 


PREFACE 


It  is  indispensable  for  the  successful  treatment  of  disease 
in  the  dental  tissues  that  the  dentist  should  acquire  as 
intimate  a  knowledge  of  structure  as  is  essential  to  the 
physician,  and  consequently  that  a  parallel  study  of  his- 
tology should  be  followed.  The  development  of  biology  has 
placed  histolog}'  at  the  basis  of  all  the  medical  sciences,  for 
as,  in  the  last  analysis,  all  physiology  is  cell  physiology,  and 
all  pathology  is  cell  pathology,  a  knowledge  of  structure 
and  function  is  necessary  for  an  intelligent  conception  of 
the  workings  of  the  animal  body  in  health  and  for  the 
restoration  of  normal  function  when  impaired  by  disease. 

Yet  as  recently  as  fifteen  years  ago,  when  the  author 
began  teaching  Dental  Histology  in  the  Northwestern 
University,  the  subject  was  comparatively  new  in  the  cur- 
riculum, and  was  considered  rather  unimportant  and  as 
having  little  practical  value.  It  would  be  impossible  to 
give  adequate  acknowledgment  to  the  help  received  from 
Dr.  G.  V.  Black  in  developing  the  course.  Every  detail 
was  worked  out  in  the  closest  cooperation  with  him,  and 
for  years  he  guided  and  directed  the  work. 

The  object  of  a  course  in  general  and  special  histology 
suited  to  the  needs  of  dental  students  is  to  convey  a  definite 
knowledge  of  the  activities  of  these  parts  of  the  human  bod}' 
in  terms  of  tissues  and  cells.  This  is  the  basis  of  every  prac- 
tical procedure.  The  structure  of  the  enamel  and  dentine  is 
obviously  the  starting  point  in  handling  these  tissues  and 
in  the  preparation  of  cavities,  and  the  structure  and  function 
of  the  pulp,  the  bone,  the  periosteum,  and  the  peridental 
membrane  are  similarly  the  basis  for  an  understanding  of 


vi  PREFACE 

their  pathology  and  treatment.  The  study  of  the  enamel  in 
relation  to  cavity  preparation  has  proved  to  be  of  the  greatest 
value  not  only  in  forming  better  cavity  walls,  but  also  in 
facilitating  operation.  For  this  purpose  it  is  necessary  to 
understand  both  the  structure  of  the  enamel  in  itself,  and 
the  arrangement  of  the  structural  elements  in  relation  to  the 
tooth  crown.  To  accomplish  this,  sections  cut  through  the 
crown  in  various  planes  must  be  studied  and  their  relation 
to  it  kept  in  mind.  The  modern  dentist  while  looking  at 
the  surface  of  a  tooth  must  think  of  the  enamel  in  terms  of 
its  structural  elements,  and  use  this  knowledge  in  handling 
the  tissue.  In  the  following  pages  the  enamel  is  studied 
primarily  in  relation  to  operative  dentistry. 

The  chapters  on  the  pulp,  peridental  membrane,  and 
periosteum  are  likewise  intended  to  emphasize  the  relation 
of  structure  to  function,  and  to  impress  the  idea  that  the 
treatment  of  disease  in  these  tissues  is  in  every  instance  a 
biological  problem.  In  forming  true  conceptions  of  caries 
and  necrosis,  a  knowledge  of  the  intercellular  substances  and 
their  relation  to  the  cells  in  the  structure  and  function  of 
tissues  is  necessary.  A  chapter  has,  therefore,  been  devoted 
to  this  subject.  The  study  of  the  structure  and  development 
of  bone  has  very  greatly  modified  treatment  in  orthodontia, 
as  it  is  now  recognized  that  in  all  movements  of  the  teeth 
the  results  are  accomplished  by  tissue  changes  under  the 
influence  of  mechanical  stimuli. 

Though  there  are  many  good  books  on  general  histology, 
they  are  not  fully  adapted  to  the  special  needs  of  the  dental 
curriculum.  The  author  has  accordingly  felt  that  teachers 
and  students  of  dentistry  might  find  some  advantage  in  a 
work  covering  the  subject  from  their  own  standpoint,  and 
embodying  the  results  of  his  experience  in  teaching  as  well 
as  in  research.  In  a  word,  this  volume  has  been  planned 
primarily  as  a  text-book  for  use  in  dental  schools,  and  it 
aims  to  provide  students  with  a  didactic  text  and  teachers 
with  a  course  to  follow.  It  contains  directions  for  twenty- 
two  days  of  laboratory  work,  and  an  appendix  giving  tech- 
nical methods  for  preparation  of  material  for  the  classes. 


PREFACE  vil 

It  suggests  many  fields  for  ot-iginal  investigation,  and  tech- 
nical directions  which  would  enable  any  man  to  begin  such 
work.  Let  us  hope  that  the  benefits  certain  to  result  from 
discoveries  still  to  be  made  will  lead  some  students  and 
practitioners  to  interest  themselves  in  this  inviting  field. 

Most  of  the  illustrations  are  from  the  author's  own  nega- 
tives. Those  on  the  relation  of  enamel  structure  to  cavity 
walls  are  new  as  well  as  original  in  plan.  The  drawings 
illustrating  the  periosteum  and  pathological  conditions  of 
the  pulp  were  made  by  Dr.  G.  V.  Black,  and  he  has  writ- 
ten the  chapter  on  his  machine  for  making  ground  sections 
and  the  technique  of  its  use.  Some  illustrations  are  taken 
from  other  works,  and  these  are  duly  credited  in  every 
instance.  Thanks  are  also  due  to  Dr.  Louis  Schmidt,  of  the 
Rockefeller  Institute,  who  made  the  colored  plates  from  the 
author's  specimens,  and  to  A.  B.  Streetdain,  of  the  University 
of  Chicago,  for  his  work  on  some  difficult  diagrams. 

Finally,  the  author  wishes  to  thank  his  publishers  for  their 
pains  and  patience  in  carrying  out  his  wishes. 

F.  B.  N. 

Chicago,  1912. 


CONTENTS 


INTRODUCTION 17 

CHAPTER  I 
Homologies 19 

CHAPTER  II 
The  Dextal  Tissues 28 

CHAPTER  HI 
The  Examel 38 

CHAPTER  IV 
The  Structural  Elemexts  of  the  Examel 43 

CHAPTER  V 
Characteristics  of  the  Examel  Tissue 52 

CHAPTER  VI 

The  Directiox  of  the  Examel  Rods  ix^  the  Tooth  Crowx     .       65 

CHAPTER  VII 

The  Relatiox  of  the  Structure  to  the  Cuttixg  of  the 

Examel  , 73 

CHAPTER  VIII 

The  Structural  Requiremexts  for  Stroxg  ExNTamel  Walls    .       80 

CHAPTER  IX 
The  Preparatiox  of  Typical  Examel  Walls 89 

CHAPTER  X 
Structural  Defects  ix  the  Enamel  .,.,....     107 


X  CONTENTS 

CHAPTER  XI 
Special  Areas  of  Weakness  for  Enamel  Margins       .      .      .     124 

CHAPTER  XII 
The  Effect  of  Caries  on  the  Structure  of  the  Enamel  .      .     143 

CHAPTER  XIII 
The  Dentine 167 

CHAPTER  XIV 
The  Cementum 188 

CHAPTER  XV 
Dental  Pulp 201 

CHAPTER  XVI 
Structural  Changes  in  the  Pathology  of  the  Pulp  .      .      .     219 

CHAPTER  XVII 
Intercellular  Substances 236 

CHAPTER  XVIII 
Bone 247 

CHAPTER  XIX 
Bone  Formation  and  Growth 255 

CHAPTER  XX 
Periosteum 262 

CHAPTER  XXI 
The  Attachment  of  the  Teeth 271 

CHAPTER  XXII 
Peridental  Membrane 279 

CHAPTER  XXIII 

The  Cellular  Elements  of  the  Peridental  Membrane.      .     294 

CHAPTER  XXIV 
The  Mouth  Cavity 323 


CONTENTS  XI 

CHAPTER  XXV 

Biological  Considerations  Fundamental  to  Embryology     .     335 

CHAPTER  XXVI 
Early  Stages  of  Embryology 340 

CHAPTER  XXVII 

The  Development  of  the  Tooth  Germ 362 

CHAPTER  XXVIII 

Th:e  Relation  of  the  Teeth  to  the  Development  of  the 

Face 374 


PART  II 

DIRECTIONS   FOR   LABORATORY  WORK 

(Twenty-four  Periods  in  the  Laboratory) 

Period  I 429 

Period  II 429 

Period  III 432 

Period  IV 435 

Period  V 435 

Period  VI 437 

Period  VII 439 

Period  VIII 440 

Period  IX 440 

Period  X 441 

Period  XI 442 

Period  XII 442 

Period  XIII 443 

Period  XIV 444 

Period  XV 445 

Period  XVI 445 

Period  XVII 446 

Period  XVIII 446 

Period  XIX 447 

Period  XX 448 

Period  XXI 449 

Period  XXII 450 

Period  XXIII 451 

Period  XXIV >  442 


xii  CONTENTS 

APPENDIX 

CHAPTER  I 

The  Grinding  of  Microscopic  Specimens,  using  the  Grind- 
ing Machine 453 

CHAPTER  II 
The  Theory  of  Histological  Technique 478 

CHAPTER  III 

General  Histological  Methods 485 

CHAPTER  IV 
Fixing  Agents  and  Staining  Solutions 496 


DENTAL  HISTOLOGY 


INTRODUCTION 

The  development  in  knowledge  of  the  cell  has  had  a  most 
profound  effect  upon  the  entire  practice  of  medicine;  in 
fact,  the  progress  of  modern  medicine  has  dated  from  the 
studies  of  cell  biology,  the  germ  theory  of  disease  being 
only  one  of  the  phases  of  this  development.  In  terms  of  the 
cell  theory  the  functions  of  the  body  are  but  the  manifest 
expression  of  the  -activities  of  thousands  or  millions  of  more 
or  less  independent  but  correlated  centres  of  activity.  If 
these  centres  or  cells  perform  their  functions  correctly,  the 
functions  of  the  body  are  normal,  but  if  they  fail  to  perform 
their  office  or  work  abnormally,  the  functions  of  the  body 
are  perverted.  In  the  last  analysis,  then,  all  physiology  is 
cell  physiology,  all  pathology  cell  pathology.  To  modern 
medicine,  histology,  or  the  cell  structure  of  the  organs  and 
tissues  of  the  body,  together  with  cell  physiology,  is  the 
rational  foundation  of  all  practice.  This  is  as  true  for  the 
dentist  as  for  the  physician  in  regard  to  the  soft  tissues  of 
the  mouth  and  teeth  that  he  is  called  upon  to  handle.  With 
caries  of  the  teeth,  the  disease  which  most  demands  the 
attention  of  the  dentist,  the  case  is  somewhat  different. 
Caries  of  the  teeth  is  an  active  destruction,  by  outside 
agencies,  of  a  formed  material  which  is  the  result  of  cell 
activity,  the  teeth  themselves  being  passive.  The  cellular 
activities  of  organs  and  tissues  of  the  body  may  have  an 
influence,  but  this  is  only  in  producing  those  conditions  of 
environment  which  render  the  activities  of  the  destructive 


18  DENTAL  HISTOLOGY 

agent  efficient  in  their  action  upon  the  tooth  tissues.  Though 
the  dental  tissues  are  passive,  the  phenomena  of  caries  can 
only  be  understood  when  the  structure  of  the  tissues  is 
understood,  and  not  only  must  the  treatment  be  based  upon 
knowledge  of  the  structure  of  the  tissues,  but  the  mechanical 
execution  of  the  treatment  is  facilitated  by  that  knowledge 
of  structure. 

In  the  preparation  of  cavities,  the  arrangement  of  the 
enamel  wall  is  determined  by  the  knowledge  of  the  direction 
of  the  enamel  prisms  in  that  locality,  and  to  a  certain  extent 
the  position  of  cavity  margins  must  be  governed  by  the 
knowledge  of  the  structure  of  the  enamel.  In  the  execution 
of  the  work  a  minute  knowledge  of  the  direction  of  enamel 
rods  becomes  the  most  important  element  in  rapidity  and 
success  of  operation.  The  longer  the  author  studies  and 
teaches  the  structure  of  the  enamel  in  its  relation  to  the 
structure  and  preparation  of  enamel  walls,  the  more  he  finds 
himself  using  this  knowledge  at  the  chair  in  daily  operations. 
He  believes  that  nothing  will  do  more  to  increase  facility, 
rapidity,  and  success  of  operation  than  a  close  study  of  the 
enamel  structure. 

All  tissues  are  made  up  of  two  structural  elements — cells 
and  intercellular  substances.  The  cells  give  the  vital  char- 
acteristics, the  intercellular  substances  the  physical  character. 
The  cells  are  the  active  living  elements,  the  intercellular 
substances  are  formed  materials  produced  by  the  activity 
of  the  cells,  and  more  or  less  dependent  upon  them  to  main- 
tain their  quality,  but  they  possess  no  vital  properties.  They 
surround  and  support  the  cells,  and  the  physical  character- 
istics are  given  by  them.  An  understanding  of  the  relation 
of  cells  and  intercellular  substances  in  the  structure  and 
function  of  tissues  is  absolutely  fundamental  to  a  study  of 
dental  histology,  and  should  be  acquired  in  a  thorough  study 
of  general  histology  before  the  subject  is  undertaken. 


CHAPTER  I 

HOMOLOGIES 

Exoskeleton. — In  studying  the  organization  of  animal  forms 
they  are  found,  very  early  in  the  evolutionary  stages,  to  de- 
velop some  sort  of  a  framework,  or  skeleton,  to  support  and 
protect  the  creature.  In  the  lower  and  earlier  forms  this 
framework  is  formed  entirely  of  some  sort  of  shell  upon  the 
outside  of  the  creature,  and  consequently  is  called  an  exo- 
skeleton. This  may  be  either  horny  or  chitinous  in  nature, 
as  in  the  insects,  crabs,  etc.,  or  it  may  be  calcified,  as  in  the 
shell-fish,  or  it  may  be  both.  The  exoskeleton  serves  not 
only  as  a  supporting  framework,  but  also  as  a  protection. 

Endoskeleton. — In  the  higher  forms  an  internal  framework, 
or  endoskeleton,  is  developed,  which  forms  the  scaffolding  to 
support  the  creature,  but  does  not  act  as  a  protection.  In  the 
first  place,  this  is  of  cartilage,  but  may  be  changed  into  bone. 
In  lower  forms  of  animals  it  remains  always  cartilage.  In 
man  the  cartilage  is  partly  converted  into  bone,  all  of  the 
bones  of  the  endoskeleton  being  preceded  by  cartilage. 

The  first  trace  of  the  endoskeleton  is  found  in  the  lowest 
form  of  vertebrate,  the  Amphioxus  or  Lancit,  the  lowest  form 
of  fish,  and  appears  as  a  rod  or  notochord  in  the  dorsal 
region.  There  is  also  an  important  difference  in  the  nervous 
organization  (Figs.  1  and  2).  In  the  invertebrate  the  nervous 
system  is  represented  by  a  larger  or  smaller  ganglion  in  the 
anterior  or  head  end,  corresponding  to  the  brain;  this  is 
dorsal  to  the  alimentary  canal.  From  this  a  ring  passes 
around  the  anterior  end  of  the  alimentary  canal  and  unites 
with  a  chain  of  ganglia  ventral  to  it.  The  nervous  system 
of  the  invertebrate  then  is,  with  the  exception  of  the  brain 
ganglia,  ventral  to  the  alimentary  canal,  and  corresponds  to 


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22  HOMOLOGIES 

the  sympathetic  system  in  higher  animals.  It  will  be  noted 
that  this  arrangement  puts  the  nervous  system,  which  controls 
the  activity  of  the  individual,  in  the  most  protected  position. 
The  invertebrate  crawling  upon  the  ground  is  subject  to 
attack  or  injury  from  above,  but  it  may  be  cut  almost  in 
two  before  the  nervous  system  is  reached. 

In  the  vertebrate  the  central  nervous  system  appears  as  a 
chain  of  ganglia  dorsal  to  the  alimentary  canal  and  notochord 
(Fig.  2).  This  difference  is  significant,  and  may  be  expressed 
roughly  in  this  way :  The  invertebrate  framework  is  an  out- 
side protecting  shell,  upon  which  the  creature  depends  for 
protection.  The  vertebrate  framework  is  an  internal  struc- 
ture to  facilitate  motion  and  give  support,  and  is  accompanied 
by  a  development  of  the  nervous  organization,  so  that  the 
creature  protects  itself  by  more  rapid  motion.  In  the  inverte- 
brate the  digestive  system  is  above  or  dorsal  to  the  nervous 
system,  in  the  vertebrate  the  nervous  system  is  in  the  upper 
position,  both  structurally  and  functionally. 

In  ascending  in  the  scale  of  organization  the  endoskeleton 
increases  in  importance  and  development,  while  the  exo- 
skeleton  decreases  in  importance  and  development. 

From  the  standpoint  of  comparative  anatomy  the  teeth 
are  not  a  part  of  the  osseous  system,  but  appendages  of  the 
skin,  and  are  to  be  compared  with  such  structures  in  the  body 
as  the  hair  and  the  nails.  The  teeth  aie  a  part  of  the  exo- 
skeleton,  and  their  relation  to  the  bones  of  the  endoskeleton 
is  entirely  secondary  for  the  purpose  of  strength,  the  bone 
growing  up  around  the  tooth  to  support  it.  When  the  skin 
of  such  an  animal  as  the  shark  is  examined,  the  entire  sur- 
face is  found  covered  with  small  calcified  bodies,  which  are 
really  small,  simple,  cone-shaped  teeth.  From  the  stand- 
point of  development  the  mouth  cavity  is  to  be  regarded  as 
a  part  of  the  outside  surface  of  the  body  which  has  been 
enclosed  by  the  development  of  neighboring  parts,  and  the 
dermal  scales,  or  rudimentary  teeth,  which  are  found  in  the 
skin  covering  the  arches  forming  the  jaws,  have  undergone 
special  development  for  the  purpose  of  seizing  and  masti- 
cating the  animal's  food.    In  the  simplest  forms  there  is  only 


HOMOLOGY  AND  ANALOGY 


23 


a  development  in  size  and  shape  of  these  scales,  and  they 
are  supported  only  by  the  connective  tissue  which  underlies 
the  skin.  These  teeth  are  easily  torn  off  in  the  attempt  to 
hold  a  resisting  prey,  and  in  the  shark  (Fig.  3)  they  are 
continually  being  replaced  by  new  ones.  In  the  more 
highly  developed  forms,  the  bone  forming  the  jaw  grows 
upward  around  the  bases  of  these  scale-like  teeth  to  support 
them  more  firmlv  and  render  them  more  useful. 


Fig.  3 


Shark's  skull  (Lamna  coinubica),  showing  succession  of  teeth. 

Homology  and  Analogy. — In  biology  structures  that  are 
similar  in  formation  and  origin  are  called  homologous. 
Structures  that  are  similar  in  function  are  called  analogous. 
A  structure  or  organ  may  be  both  homologous  and  analo- 
gous to  another,  but  not  necessarily  so.  For  instance,  the 
wing  of  a  fly  is  analogous  to  the  wing  of  a  bird,  because  they 
are  used  for  the  same  purpose,  but  they  are  not  homologous. 
The  wing  of  a  bat  and  the  wing  of  a  bird  are  both  analogous 
and  homologous,  being  used  for  the   same  purpose,  and 


24 


HOMOLOGIES 


having  similar  structure  and  origin.  The  arm  of  man  is 
homologous  to  the  wing  of  a  bird  but  not  analogous  to  it. 
The  jaws  of  a  crab  or  beetle  are  analogous  to  the  jaws  of  man, 
but  they  are  not  homologous  structures,  as  the  jaws  of  the 
crabs  and  insects  are  modified  legs.    The  teeth  are  said  to 


Fig.  4 


,-^5^3^???; 


Development  of  the  hair:  Sc,  stratum  corneum;  SM.  stratum  malpighii;  C,  derma; 
F,  follicle;  Dr,  sebaceous  gland;  CZ,  central,  PZ,  peripheral  zone  of  hair  germ;  HK, 
hair  knob;  P,  beginning  the  formation  ol  the  hair  papilla;  P',  same  in  a  later  stage  of 
development  when  it  has  become  vascular.  (Wiedersheim,  Comparative  Anatomy  of 
Vertebrates.) 

be  homologous  to  the  dermal  scales  of  certain  fishes,  and  to 
the  appendages  of  the  skin,  such  as  the  hair  and  nails,  because 
they  are  similar  in  structure  and  origin  (Plate  I). 

Comparison  of  Structure. — If  the  tooth  is  compared  with  the 
hair  in  this  way  this  will  be  better  understood.      The  hair 


PLATE   I 


Comparison  of  Structure  of  Tooth  and  Hair. 


COMPARISON  OF  ORIGIN 


25 


may  be  considered  as  a  horny 
structure  composed  of  epithelial 
cells  resting  upon  a  papilla  of 
connective  tissue.  The  tooth 
may  be  considered  a  calcified 
structure,  formed  by  epithelial 
cells,  resting  upon  a  papilla  of 
connective  tissue,  which  is  also 
partially  calcified. 

Comparison  of  Origin. — From  a 
study  of  the  development  of  the 
tooth  and  the  hair,  the  similarity 
of  their  origin  and  structure  be- 
comes more  apparent. 

The  first  step  in  the  develop- 
ment of  the  hair  is  a  thickening 
of  the  epithelium  at  a  point,  the 
epithelial  cells  multiplying  and 
growing  down  into  the  connective 
tissue  below,  so  as  to  make  a 
two-layered  bag  or  cap,  the  con- 
nective tissue  groAving  up  in  the 
form  of  a  cone-shaped  papilla 
into  the  cavity  of  the  cap  (Fig. 
4).  The  epithelial  cells  of  the 
inner  layer,  next  to  the  connec- 
tive tissue,  multiply  rapidly  and 
develop  horny  material  and  are 
pushed  out  from  the  surface  of 
the  skin  as  the  shaft  of  the  hair. 


Diugram  to  illustrate  development  of  a 
tooth:  A.  inner  layer  of  enamel  germ;  B, 
outer  layer;  C,  remains  of  intermediate  cells; 
D,  dentine;  Z)./,  dental  lamina;  ^.epithelium; 
E.G,  enamel  germ;  En,  enamel;  F,  dental 
furrow;  L.D,  labiodental  furrow;  M,  con- 
nective-tissue cells;  O,  odontoblasts:  P,  den- 
tine papillaf  R.G,  reserve  germ;  V,  blood- 
vessel.     (Cunningham's  Anatomy.) 


Fig.  5 


III  f'> 


V  \  ■ 


26 


HOMOLOGIES 


In  the  development  of  the  tooth  there  is  at  first  a  thicken- 
ing of  the  epithehum,  and  a  mass  of  epithehal  cells  like  that 
forming  the  hair,  hut  larger,  grows  down  into  the  connective 
tissue  (Fig.  5).  This  becomes  bulbous,  then  invaginated, 
forming  a  two-layered  cap.  The  two  layers  are  at  first  per- 
fect and  are  farther  from  the  surface  than  the  epithelial 


Fig.  6 


Changes  in  the  mandible  with 


buccal  and  lingual  view. 


structure  which  develops  the  hair.  A  cone-shaped  papilla 
of  connective  tissue,  the  dental  papilla,  grows  up  into  the 
cavity  of  the  epithehal  organ  corresponding  to  the  bulb  of 
the  hair. 

The  inner  layer  of  epithelial  cells  produce  the  enamel,  the 
outer  layer  of  connective-tissue  cells,  covering  the  connective- 


RELATION   TO   THE  BONE  27 

tissue  papilla,  develop  the  dentine,  leaving  the  pulp  inside  as 
the  remains  of  the  dental  papilla. 

Relation  to  the  Bone. — The  relation  of  the  bones  of  the  jaws 
to  the  teeth  is  entirely  secondary  and  transient.  They  grow 
up  around  the  roots  of  the  teeth  to  support  them,  and  are 
destroyed  and  removed  with  the  loss  of  the  teeth  or  the 
cessation  of  their  function.  In  this  way  the  development  of 
the  alveolar  process  appears  around  the  roots  of  the  tem- 
porary teeth.  All  this  bone  surrounding  their  roots  is  ab- 
sorbed and  removed  with  the  loss  of  the  temporary  denti- 
tion, and  a  new  alveolar  process  grows  up  around  the  roots 
of  the  permanent  teeth  as  they  are  formed.  This  develop- 
ment of  bone  around  the  roots  of  the  teeth  leads  to  the 
changes  in  the  shape  of  the  body  of  the  lower  jaw,  increasing 
the  thickness  from  the  mental  foramen  and  the  inferior  dental 
canal  upward  (Fig.  6) .  When  the  teeth  are  finally  lost  this 
bone  is  again  removed  and  the  body  of  the  jaw  is  reduced  in 
thickness  from  above  downward.  These  phenomena  have 
an  important  bearing  upon  the  causes  and  treatment  of 
diseased  conditions  of  the  teeth,  particularly  those  which 
involve  the  supporting  tissues. 


CHAPTER  II 

THE   DENTAL   TISSUES 

Study  of  the  structure  of  the  teeth  shows  that  all  teeth, 
from  the  simplest  to  the  most  complex,  are  composed  of  but 
four  tissues — enamel,  dentine,  cementum,  and  the  pulp,  or 
formative  tissue  of  the  dentine. 

Even  the  simplest  placoid  scales,  as  found  in  the  skin  of 
the  shark  and  dog-fish,  contain  these  four  tissues.  In  many 
of  the  specialized  forms  of  teeth  some  of  these  tissues  may 
be  absent.  For  instance,  in  the  bony  fishes  the  teeth  are 
fastened  to  the  bone  by  an  interlocking  of  bone  and  dentine, 
forming  an  ankylosed  attachment,  and  the  cementum  is 
absent;  but  in  some  of  these  there  is  also  a  slight  formation 
of  cementum.  In  the  tusks  of  elephants  during  the  func- 
tional period  the  dentine  is  not  covered  by  enamel,  but 
when  the  tusk  first  erupted  there  was  a  slight  enamel  cap, 
which  was  at  once  broken  or  worn  off.  In  many  instances 
the  enamel  seems  to  be  entirely  absent,  and  for  that  reason 
it  has  sometimes  been  called  the  most  inconstant  of  the 
dental  tissues,  but  in  every  case  in  which  the  development  of 
the  tooth  has  been  studied  an  enamel  organ  has  been  found. 
It  is  probably  much  more  nearly  correct  to  consider  that  in 
all  cases  enamel  is  formed,  but  that  it  may  be  so  thin  and 
transparent  as  to  be  very  difficult  to  recognize,  and  very  soon 
may  be  entirely  lost. 

• 

FUNCTIONS  OF  THE   DENTAL  TISSUES 

The  Enamel. — The  enamel  forms  a  hard  protecting  surface 
or  cap  especially  adapted  to  resist  abrasion.  It  is  the  hardest 
animal  tissue,  but  brittle  and  inelastic,  and  dependent  upon 


FUNCTIONS  OF  THE  DENTAL  TISSUES  29 

the  support  of  the  elastic  dentine  for  strength.  Its  func- 
tion is  to  resist  the  abrasion  of  friction.  Its  arrangement  in 
many  instances  is  found  specially  modified  for  this  purpose. 
The  Dentine. — The  dentine  is  the  strong  elastic  tissue  form- 
ing the  great  mass  of  the  tooth,  and  gives  to  it  its  strength. 
Teeth  that  are  subjected  to  stress  and  force  are  often  made 
up  of  dentine  without  enamel.  If,  for  instance,  the  tusks 
of  the  elephant  used  for  such  purposes  as  tearing  down 
branches,  spading  up  the  ground,  and  so  on,  were  made 
up  entirely  of  enamel,  they  would  break  off  the  first  time 
they  were  locked  in  the  branches  or  driven  into  the  ground, 
but  the  elastic  dentine  gives  and  bends  and  will  stand  great 
stress.  The  teeth  of  many  animals  which  use  their  tusks  in 
fighting  are  constructed  on  the  same  plan. 

The  Cementum. — The  cementum  furnishes  attachment  for 
the  connective-tissue  fibers  which  fasten  the  tooth  to  the 
bone  or  surrounding  tissues.  It  is  formed  on  the  enamel  and 
dentine  both  before  and  after  the  eruption  of  the  teeth. 
The  formation  of  the  cementum  on  the  surface  of  the  root 
fastens  the  surrounding  connective-tissue  fibers  to  the 
tooth.  The  fibers  are  calcified  along  with  the  matrix  of  the 
cementum  which  is  built  up  around  them.  These  fibres  in 
man  and  the  higher  animals  extend  to  the  bone  and  the 
surrounding  tissues  and  support  the  teeth  against  the  forces 
of  occlusion,  and  hold  the  surrounding  tissues  in  proper 
relation  to  the  teeth.  The  function  of  the  cementum  is, 
therefore,  to  attach  the  connective  tissue  fibers  to  the  surface 
of  the  root. 

The  Pulp. — The  pulp  is  the  remains  of  the  formative  organ  of 
the  dentine.  In  teeth  of  continuous  growth  it  remains  actively 
functional  throughout  the  life  of  the  tooth,  but  in  teeth  of 
limited  grow^th,  after  the  typical  development  of  dentine, 
it  becomes  functional  again  only  in  response  to  irritations, 
which,  however,  may  be  local  or  reflex.  The  pulp  performs 
two  functions — a  vital  function,  the  formation  of  dentine,  and 
a  sensory  function,  the  response  to  thermal  change. 

Summaxy.  —  The  dental  tissues,  i.  e.,  enamel,  dentine, 
cementum,  and  pulp,  are  so  called  not  simply  because  they 


30  THE  DENTAL  TISSUES 

are  found  in  the  human  teeth,  but  because  all  teeth  are 
composed  of  these  four  tissues. 

It  is  true  that  in  comparative  dental  histology  consider- 
able difference  exists  in  the  microscopic  structure  of  these 
tissues  from  the  teeth  of  different  animals,  but  certain 
characteristics  are  very  persistent  and  quite  characteristic 
of  each. 


DISTRIBUTION  OF  THE  DENTAL  TISSUES 

The  arrangement  and  distribution  of  the  dental  tissues 
in  the  structure  of  the  human  teeth  is  best  studied  in  ground 
sections  cut  longitudinally  through  the  entire  tooth  (Plate  II), 
and  series  of  transverse  sections  cut  through  the  roots.  For 
this  purpose  the  sections  should  not  be  too  thin  (from  10 
to  20  microns).  For  the  study  of  the  arrangement  of  the 
cementum  and  dentine  in  the  roots  at  least  three  transverse 
sections  should  be  ground  from  each  root,  one  from  the 
gingival,  one  from  the  middle,  and  one  from  the  apical  third. 

The  Enamel. — The  enamel  forms  a  cap  over  the  exposed  por- 
tion of  the  tooth.  Its  function  is  to  resist  the  abrasions  of  mas- 
tication. It  gives  the  detail  of  crown  form  to  the  tooth.  It 
extends  to  the  gingival  line,  and,  except  in  old  age,  is  covered 
in  the  gingival  portions  by  the  epithelium  of  the  gingivus, 
which  lies  in  contact  with  it  but  is  not  attached  to  it.  It  is 
thin  in  the  gingival  portion  and  is  normally  overlapped  slightly 
by  the  cementum  at  the  gingival  line.  It  extends  farther 
apically  on  the  labial  and  lingual,  and  buccal  and  lingual, 
than  upon  the  proximal  surfaces,  especially  on  the  incisors, 
cuspids,  and  bicuspids.  It  is  thickest  in  the  occlusal  third 
of  the  axial  surfaces,  and  on  the  occlusal  surfaces  of  the 
molars  and  bicuspids,  especially  over  the  cusps.  In  the 
incisors  and  cuspids  it  is  thickest  in  the  occlusal  third  on 
the  labial  and  over  the  marginal  ridges  on  the  lingual  and  the 
dento-enamel  junction,  which,  though  not  parallel  with  the 
surface  of  the  enamel,  is  usually  curved  in  the  same  direction. 

In  the  molars  and  bicuspids  the  dento-enamel  junction  in 


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DISTRIBUTION  OF   THE  DENTAL   TISSUES         31 

the  occlusal    thirds  on  the  buccal  and   lingual  is  usually 
curved  in  the  opposite  direction.    That  is,  while  the  surface 


Fin. 


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32  THE  DENTAL   TISSUES 

of  the  enamel  is  convex,  the  surface  of  the  dentine  is  concave. 
It  will  be  seen  that  this  not  only  gives  a  greater  thickness 
to  the  enamel  in  the  region  which  will  resist  abrasion, 
but  also  gives  it  a  firmer  seat  upon  the  dentine.  (Study 
illustrations  in  Chapter  X.)  The  dento-enamel  junction  is 
seldom  a  smooth,  even  surface,  but  will  appear  scalloped 
in  sections,  projections  of  dentine  extending  between 
projections  of  enamel  (Fig.  7).  In  three  dimensions  this 
means  that  rounded  projections  of  the  enamel  rest  in 
rounded  depressions  of  the  dentine  surface,  and  pointed 
projections  of  the  dentine  extend  between  the  rounded  pro- 
jections of  the  enamel.  This  is  similar  but  much  less  marked 
than  the  interlocking  of  the  papilla  of  connective  tissue  with 
the  projections  of  the  Malpighian  layer  of  stratified  squamous 
epithelium  of  the  skin  and  mucous  membrane.  In  some 
cases  these  projections  of  dentine  into  the  enamel  may  be 
quite  marked.  This  scalloping  of  the  dento-enamel  junc- 
tion gives  a  stronger  attachment  of  the  enamel  to  the  den- 
tine, and  accounts,  partially  at  least,  for  the  difference  that 
is  observed  in  the  ease  with  which  enamel  can  be  removed 
from  the  dentine  in  the  preparation  of  roots  for  crowns. 
Where  the  two  tissues  join  with  smooth  surfaces  the  enamel 
can  be  comparatively  easily  cleaved  away;  Avhere  the 
scalloping  is  marked  it  is  removed  with  much  greater 
difficulty. 

The  Dentine. — The  dentine  gives  the  strength  to  the  tooth. 
This  should  never  be  lost  sight  of  in  operations,  and  sound 
dentine  should  always  be  conserved  to  the  greatest  possible 
extent  in  the  preparation  of  cavities.  That  the  function  of  the 
dentine  is  to  give  strength  will  be  seen  more  clearly  from  a 
comparative  study  of  teeth  modified  for  special  functions. 
The  dentine  forms  the  greatest  mass  of  the  tooth,  the  type 
form  being  determined  by  it.  The  cusps  and  ridges,  although 
different  in  form,  are  still  represented  in  the  dentine  as  well 
as  the  number  and  shape  of  the  roots,  while  the  detail  of 
the  form  of  the  roots  is  modified  by  the  addition  of  the 
cementum  on  the  surface. 

The  dentine  forms  a  layer  of  comparatively  even  thickness 


DISTRIBUTION  OF  THE  DENTAL   TISSUES         33 

surrounding  the  central  cavity  or  pulp  chamber,  which  is 
occupied  by  the  formative  organ.  From  this  cavity  a  great 
number  of  small  tubules  extend  through  the  calcified  dentine 
matrix  to  the  surface  under  the  enamel  and  cementum.  In 
the  crown  portion  the  course  of  these  tubules  is  characteris- 
tically curved  like  the  letter  S  or  /,  so  that  the  tubules  tend 
to  enter  the  pulp  chamber  at  right  angles  to  the  surface  and 
to  end  under  the  enamel  at  right  angles  to  the  dento-enamel 
junction  (Plate  II).  On  closer  study  these  tubule  directions 
will  be  found  to  be  more  complicated,  but  in  studying  the 
distribution  of  dentine  they  should  be  noted.  In  the  root 
portion  the  tubules  are  usually  comparatively  straight, 
that  is,  without  the  double  curve,  and  are  at  about  right 
angles  to  the  axis  of  the  canal. 

The  outer  layer  of  dentine  under  low  magnification  pre- 
sents a  peculiar  granular  appearance,  which  is  specially 
apparent  under  the  cementum.  This  is  known  as  the 
granular  layer  of  Tomes,  and  is  caused  by  irregular  spaces 
in  the  dentine  matrix  which  communicate  with  the  dentinal 
tubules. 

The  Cementum. — The  cementum  covers  the  dentine  in  the 
root  portion,  and  in  most  cases  slightly  overlaps  the  enamel  at 
the  gingival  line.  This  is  not  always  true,  for  in  some  cases  it 
just  meets  the  enamel,  and  in  others  there  is  a  space  where 
the  dentine  is  uncovered  between  the  enamel  and  the 
cementum  (Fig.  8).  It  has  not  been  positively  determined 
whether  this  can  ever  be  considered  a  normal  condition, 
and  the  author  has  some  reason  to  suppose  that  the  sec- 
tions showing  this  condition  were  from  teeth  from  which  the 
gums  had  receded  and  the  cementum  was  destroyed.  The 
sensitiveness  which  is  so  marked  in  some  cases,  where  the 
gums  have  receded  beyond  the  gingival  line,  is  probably  due 
to  the  loss  of  cementum  and  the  uncovering  of  the  granular 
layer  of  Tomes. 

The  cementum  is  thin  and  structureless  in  appearance  in 

the  gingival  portion  when  viewed  with  low  powers,  but 

becomes  thicker  in  the  apical  third.    In  the  thicker  portions 

irregular  spaces  (lacunae)  with  radiating  canals  (canaliculi) 

3 


34 


THE  DENTAL  TISSUES 


are  seen.  In  life  these  spaces  contain  living  cells  (the  cement 
corpuscles),  which  correspond  to  the  bone  corpuscles  found 
in  the  lacuna  of  bone.  Upon  the  convex  surfaces  of  the  root 
the  cementum  is  thin;  upon  the  concave  surface  it  is  thicker. 
This  increases  with  age,  and  so  the  continuous  formation  of 
cementum  tends  to  round  the  outlines  of  the  roots  and  to 


Fig.  8 


Gingival  line,  showing  the  relation  of  enamel  and  cementum. 


unite  them  where  they  approach  each  other.  The  fibers 
which  are  built  in  the  cementum  are  often  imperfectly 
calcified,  especially  where  the  layers  are  thick,  so  that  in 
the  ground  sections  they  may  often  be  easily  mistaken  for 
canals,  because  the  imperfectly  calcified  fiber  has  shrunken 
in  the  preparation. 


ADAPT  A  TION  IN  DISTRIBUTION  OF  DENTAL  TISSUES  35 


ADAPTATION  IN  THE  DISTRIBUTION  OF  DENTAL 
TISSUES 

If  the  teeth  of  mammals  are  studied  in  a  comparative  way 
many  modifications  will  be  found  in  the  relative  amount  and 
distribution  of  the  dental  tissues,  adapting  the  tooth  to  per- 
form special  functions.  A  study  of  these  modified  or  special- 
ized teeth  will  give  a  better  understanding  of  the  functions 
of  the  tissues  in  the  tooth.  The  human  tooth  may  be  taken 
as  a  type  of  omnivorous  tooth,  and  the  arrangement  and 
distribution  of  its  tissues  has  already  been  described. 

Teeth  of  Continuous  Growth. — In  many  animals  the  teeth 
or  some  special  teeth  are  developed  as  weapons  for  use  in 
fighting,  or  as  implements  to  aid  in  securing  food.  It  is 
usually  the  cuspid  teeth  that  show  this  modification,  as  in 
the  tusks  of  the  boar  and  many  species  of  the  carnivora,  the 
tusks  of  the  walrus,  and  other  examples.  In  the  case  of  the 
elephant  the  incisors  have  been  developed  in  the  same  way. 
Whenever  the  teeth  have  been  developed  in  size  for  uses 
which  require  strength  and  the  ability  to  withstand  stress 
and  strain,  the  increase  in  size  is  by  development  of  the  mass 
of  dentine,  the  enamel  often  being  entirely  lost  during  the 
functional  period.  If  these  teeth  were  composed  chiefly  of 
enamel  they  would  be  too  brittle.  These  tusks,  which,  as  in 
the  case  of  the  elephant,  sometimes  reach  a  weight  of  many 
hundreds  of  pounds,  are  usually  deeply  embedded  in  the 
bone,  and  the  concealed  portion  is  covered  with  a  layer  of 
cementum  which  attaches  the  fibers,  holding  them  to  the 
bone,  but  they  retain  a  conical  pulp  in  a  cone-shaped  pulp 
chamber  at  the  base  of  the  tooth,  which  continues  to  form 
dentine.  The  tooth  is  pushed  out  of  the  socket,  as  the 
shaft  of  the  hair  is  pushed  out,  by  the  multiplication  of 
cells  covering  the  bulb.  In  this  way  the  size  of  the  tooth  is 
maintained  as  the  exposed  and  functional  part  is  worn  off. 
Strength  and  elasticity  are  required,  therefore  the  dentine 
is   developed.       The   cementum   which   is   formed   on   the 


36  THE  DENTAL  TISSUES 

embedded  portion  for  attachment  of  fibers  is  worn  off  as  soon 
as  it  is  exposed  to  friction. 

Chisel  Teeth. — The  incisors  of  the  rodents,  as  rats,  mice, 
squirrels,  and  beavers,  present  an  interesting  modification  for 
a  special  function.  These  teeth  are  used  as  chisels  for  cutting 
hard  substances,  as  wood,  shells  of  nuts,  etc.  Here  strength 
and  hardness  are  required.  The  dentine  is  increased  by 
the  continual  function  of  a  conical  persistent  pulp  which 
continues  to  form  dentine,  and  the  enamel  organ  is  carried 
down  into  the  socket,  to  the  base  of  the  dental  papilla,  on 
the  labial,  instead  of  stopping  at  the  gingival  line,  as  in  the 
human  incisors.  In  this  position  it  continues  to  build 
enamel  on  the  labial  side  of  the  dentine.  The  enamel 
rods,  instead  of  being  straight,  are  twisted  about  each 
other  in  a  complicated  fashion,  giving  the  maximum  of 
hardness.  As  the  incisors  work  against  each  other  by  the 
movements  of  the  jaw,  the  dentine  is  worn  off  on  the  lingual 
side  and  the  enamel  kept  in  the  form  of  a  chisel  edge.  There 
is  also  a  modification  of  the  temporomandibular  articulation, 
allowing  the  lower  jaw  to  move  forward  and  back  as  well  as 
up  and  down,  but  not  laterally,  so  that  the  lower  incisors 
can  be  closed  either  lingually  or  labrally  to  the  upper,  and 
in  this  way  both  the  upper  and  the  lower  incisors  are  made 
to  sharpen  each  other  in  use.  In  this  case  there  is  need  for 
both  strength  and  hardness,  and  both  dentine  and  enamel 
are  continuously  being  formed  at  the  base  of  the  tooth 
embedded  in  the  socket,  and  the  cementum  is  formed  over 
the  embedded  portions  as  the  medium  of  attachment. 

Grinding  Teeth. — In  a  grinding  tooth,  as  in  the  molar  of 
the  horse  and  cow,  and  in  a  much  more  complicated  form 
in  the  elephant,  the  three  tissues — enamel,  cementum,  and 
dentine — are  arranged  so  as  to  form,  by  the  different  rapidity 
of  abrasion,  corrugated  grinding  surfaces  like  millstones. 
The  conditions  can  be  understood  if  it  is  remembered  that 
the  cusps  in  the  dentine  are  very  high,  and  are  covered  by 
a  comparatively  thin  layer  of  enamel.  After  the  enamel  is 
formed,  and  while  the  tooth  is  embedded  in  its  crypt  in  the 
bone,  cementum  is  formed,  covering  the  surface  and  filling 


ADAPTA  TION  IN  DISTRIBUTION  OF  DENTAL  TISSUES  37 

up  the  hollows  between  the  cusps,  so  that  the  crown  when 
it  first  erupts  is  rounded,  with  only  enamel  showing  at  the 
tips  of  the  cusps.  As  soon  as  the  tooth  wears,  the  tip  of 
the  enamel  is  worn  through,  so  that  the  circumference  of  the 
crown  shows  first  cementum,  then  enamel,  then  dentine, 
then  enamel,  then  cementum,  then  enamel,  and  so  on.  The 
foldings  of  the  enamel  often  become  very  complicated,  but 
the  most  complicated  forms  can  be  understood  in  this  way. 

In  describing  the  structure  of  the  teeth  and  the  arrange- 
ment of  the  structural  elements  of  the  tissues,  directions  are 
described  with  reference  to  three  planes:  The  mesio-disto- 
axial  plane  passing  through  the  centre  of  the  crown  from 
mesial  to  distal  and  parallel  with  the  long  axis  of  the  tooth. 

The  bucco-linguo-axial  plane,  a  plane  passing  through  the 
centre  of  the  crown  from  buccal  to  lingual  and  parallel  with 
the  long  axis  of  the  tooth. 

The  horizontal  plane  at  right  angles  to  the  axial  planes. 


CHAPTER  III 

THE  ENAMEL 

The  enamel  differs  from  all  other  calcified  tissues: 

1.  In  origin. 

2.  In  degree  of  calcification. 

3.  In  relation  to  its  formative  organ. 

4.  In  the  form  of  the  structural  elements  of  the  tissue. 
It  is  well  to  emphasize  these  points  of  difference,  for 

throughout  dental  and  medical  writing,  reasoning  by 
analogy  from  bone  conditions  to  tooth  conditions,  and 
especially  to  changes  in  the  enamel,  is  often  found.  For 
instance,  the  argument  has  been  made  that  because  there 
may  be  changes  in  the  bones  in  pregnancy,  "softening"  of 
the  teeth  would  be  expected.  Many  similar  though  less 
crude  arguments  would  not  be  made  if  it  were  remembered 
that  histologically,  histogenetically,  physiologically,  and 
morphologically  the  enamel  stands  alone. 

Origin. — The  enamel  is  the  onl}^  calcified  tissue  derived 
from  the  epithelium.  All  other  calcified  tissues  are  con- 
nective tissues.  Histogenetically,  then,  the  enamel  is 
ultimately  derived  from  the  epiblastic  germ  layer,  while  all 
other  calcified  tissues  arise  from  the  mesoblast.  Thus,  even 
at  the  first  step  in  the  differentiation  of  cells,  the  enamel  is 
different  and  independent  from  bone,  cementum,  or  dentine. 
It  is  natural,  therefore,  to  find  the  enamel  differing  from 
bone  in  every  other  respect.  On  the  other  hand,  the  relation 
of  the  enamel  to  the  epithelium  becomes  more  and  more 
apparent.  For  instance,  imperfections  in  the  structure  of 
the  enamel  during  its  formation  are  most  likely  to  be  pro- 
duced by  systemic  conditions  which  affect  the  epithelium. 
The  eruptive  fevers  occurring  during  enamel  formation  often 


•       DEGREE  OF  CALCIFICATION  39 

produce  imperfections  of  structure.  Scarlet  fever  is  most 
pronounced  in  its  epithelial  effect,  causing  loss  of  skin,  loss 
of  living  epithelium  of  the  alimentary  tract,  and  often  loss 
of  hair,  and  is  likewise  most  likely  to  produce  pitted  and 
atrophied  teeth.  In  other  words,  the  same  poison  which  is 
produced  by  the  germ  of  scarlet  fever  causes  the  death  of 
epithelial  cells,  of  the  skin,  of  the  hair  bulb,  of  the  mucous 
membrane,  and  of  the  enamel  organ. 

The  most  recent  work  of  Dr.  Black  shows  the  brown  and 
mottled  enamel  of  certain  localities  to  be  found  associated 
with  greatly  freckled  skin.  Enamel,  therefore,  must  be 
considered  as  epithelial  in  origin  and  ultimately  from  the 
epiblast,  while  all  other  calcified  tissues  are  connective 
tissue  and  ultimately  of  mesoblastic  origin. 

Degree  of  Calcification. — The  enamel  is  by  far  the 
hardest  animal  tissue.  Chemically  it  is  composed  of  water, 
calcium  phosphate,  carbonate,  and  a  small  amount  of 
fluoride,  magnesium  phosphate,  and  a  trace  of  other  salts. 
Normally  it  should  contain  no  organic  matter.  Von  Bibra 
gives  the  following  analysis  : 

Calcium  phosphate  and  fluoride 89 .  82 

Calcium  carbonate 4.37 

^Magnesium  phosphate 1.34 

Other  salts 0.88 

Cartilage 3.39 

Fat 0.20 

It  is  very  difficult  to  obtain  enamel  for  chemical  analysis 
entirely  free  from  dentine,  and  small  portions  of  dentine 
clinging  to  it  are  probably  responsible  for  some  of  the 
organic  matter  given  in  the  above  analysis. 

In  all  the  older  analyses  the  enamel  was  said  to  contain 
95  to  97  per  cent,  of  inorganic  matter,  and  3  to  5  per  cent, 
of  organic  matter,  while  the  percentage  in  dentine  was  given 
as  72  per  cent,  of  inorganic  and  28  per  cent,  of  organic,  and 
in  bone  as  68  per  cent,  inorganic  and  32  per  cent,  organic 
(dry  compact  bone).  This  in  itself  shows  an  enormous 
difference  in  the  degree  of  calcification  between  enamel  and 
the  other  hard  tissues,  but  the  results  of  more  recent  work  are 


40  THE  ENAMEL 

still  more  remarkable.  In  most  of  the  original  studies  of 
the  chemical  composition,  the  enamel  was  broken  into 
small  pieces  and  dried  for  some  time  at  a  temperature 
above  the  boiling  point  of  water,  to  drive  off  all  the  moisture. 
The  dry  enamel  was  weighed  and  then  ignited,  and  the  loss 
in  weight  taken  as  the  amount  of  organic  matter.  In  1896 
Dr.  Charles  Tomes,^  of  London,  published  the  results  of  his 
chemical  analysis  of  enamel,  in  which  he  showed  that  a  large 
part  of  the  loss  of  weight  in  ignition  was  due  to  the  loss  of 
water.  He  carried  out  ignition  in  tubes  to  collect  the  products 
of  combustion,  and  found  that  between  red  and  white  heat 
from  2  to  3  per  cent,  of  water  was  given  off.  This  occurred 
suddenly  and  with  almost  explosive  violence,  blowing  large 
pieces  to  fragments.  While  this  did  not  account  entirely 
for  all  of  the  matter  previously  considered  organic,  the 
character  of  the  product  of  combustion  and  the  observation 
of  the  material  during  ignition  led  him  to  conclude  that  the 
remaining  portion  was  due  to  the  dentine  adhering  to  the 
enamel,  and  that  the  enamel  contained  not  more  than  a  trace 
of  organic  matter. 

Dr.  Leon  Williams  attacked  the  problem  from  the  micro- 
scopic and  microchemical  side,  and  was  forced  to  the  con- 
clusion that  normal  enamel  contains  no  organic  matter. 
No  trace  of  organic  matter  can  be  found  in  sections  of 
enamel  by  staining.  And  if  the  enamel  is  dissolved  by  acid 
and  the  progress  observed,  not  a  trace  of  organic  matrix 
can  be  found.  The  conclusion  is  therefore  imperative  that 
enamel  is  composed  entirely  of  inorganic  matter,  which  has 
been  deposited  and  calcified  in  the  form  of  the  tissue  by  the 
formative  cells.  In  other  words,  enamel  is  formed  material 
produced  by  cells  and  laid  down  in  a  definite  structure,  but 
it  contains  no  organic  matrix,  while  all  other  calcified  tissues 
are  composed  of  an  organic  matrix  of  ultimate  fibrous  and 
gelatin-yielding  character,  in  which  inorganic  salts  are 
deposited  in  a  weak  chemical  combination,  and  living  cells 
are  retained  in  spaces  of  the  formed  material. 

1  Journal  of  Physiology. 


RELATION  TO  THE  FORMATIVE  TISSUE  41 

If  bone  or  dentine  is  subjected  to  the  action  of  acid, 
the  combination  between  the  organic  and  inorganic  matter 
is  broken  up  and  the  inorganic  matter  dissolved,  leaving 
the  organic  portion,  which  yields  gelatin  when  boiled  in 
water,  in  the  form  of  the  original  tissue.  If  enamel  is 
treated  with  acid  the  cementing  substance  between  the  rods 
is  first  attacked  and  is  dissolved  more  rapidly,  then  the  rods 
are  attacked  from  their  sides,  and  finally  the  tissue  is  entirely 
destroyed,  leaving  no  trace  of  structure.  Apparently  the 
greater  the  dilution  of  the  acid  the  greater  will  be  the  extent 
of  the  solution  of  the  cementing  substance  before  the  rods 
are  destroyed. 

If  bone  or  dentine  is  burned  or  ignited,  the  organic 
matter  will  be  driven  off  and  the  inorganic  portion  will  be 
left  in  the  form  of  the  tissue,  still  showing  its  structure.  If 
enamel  is  ignited,  water  of  combination  and  whatever 
foreign  matter  has  clung  to  the  pieces  is  given  off,  but  the 
form  of  the  tissue  is  unchanged.  To  illustrate  the  difl^er- 
ence  by  a  crude  comparison:  Bone  matrix  may  be  likened 
to  a  piece  of  cloth  into  which  organic  salts  have  been 
deposited  until  it  has  become  stifle  and  rigid,  but  the  web 
of  the  cloth  is  still  seen.  The  salts  may  be  dissolved  out 
and  the  cloth  left,  or  the  cloth  may  be  burned  out  and  the 
salts  left.  The  enamel  may  be  compared  to  a  fossil  in  which, 
by  molecular  change,  the  organic  matter  has  been  removed 
and  inorganic  matter  substituted,  so  that  no  organic  matter 
remains,  but  the  structure  is  preserved.  If  the  inorganic 
salts  were  dissolved,  no  trace  of  structure  would  remain. 
On  the  other  hand,  by  ignition,  nothing  but  water  can  be 
driven  oft'. 

Relation  to  the  Formative  Tissue. — The  enamel  is  produced 
by  epithelial  cells,  which  are  lost  and  destroyed  after  the 
tissue  is  completed.  Any  such  thing,  therefore,  as  a  vital 
change  in  the  tissue  is  biologically  unthinkable.  After  the 
enamel  is  formed  it  can  be  changed  only  by  chemical  and 
physical  action  of  its  environment. 

All  other  calcified  tissues  are  formed  by  connective  tissue, 
and  remain  in  vital  relation  with  connective  tissue  of  undifter- 


42  THE  ENAMEL 

entiated  character.  Bone  and  dentine  matrix  are,  therefore, 
simply  calcified  intercellular  substances  containing  living 
cells  in  the  spaces  of  the  matrix,  which  maintain  its  chemical 
quality.  A  change  in  the  character  or  amount  of  the  matrix 
might  possibl}^,  therefore,  be  brought  about  by  the  vital 
activit}^  of  these  cells.  Moreover,  the  formed  matrix  is 
always  in  vital  relation  with  undifferentiated  connective 
tissue,  which  may  at  any  time  destroy  or  rebuild  it.  There 
is,  therefore,  no  basis  for  comparison  between  pathologic 
conditions  of  bone  and  enamel. 

The  Form  of  the  Structural  Elements. — The  enamel  is  made 
up  of  prismatic  rods  of  inorganic  matter,  held  together 
by  an  inorganic  cementing  substance.  All  other  calcified 
tissues  are  made  up  of  fibrous  intercellular  substance,  con- 
taining inorganic  salts  and  usually  arranged  in  layers.  The 
structure  of  the  enamel  differs  so  greatly  from  all  other 
calcified  tissues  that  it  is  difficult  to  compare  them  briefly. 


CHAPTER  IV 

THE   STRUCTURAL  ELEMENTS   OF   THE   ENAMEL 

The  enamel  is  composed  of  two  structural  elements: 

1.  The  enamel  rods  or  prisms,  sometimes  called  enamel 

fibers. 

2.  The  interprismatic,  or  cementing  substance. 

Fig.  9 


£na 


JO  X) 


Enamel  Rods.— The  enamel  rods  are  long  slender  prismatic 
rods  irregularly  five  or  sLx  sided  and  alternately  expanded  and 
constricted  throughout  their  length  (Figs.  9  and  10).  They 
are  from  three  and  four-tenths  to  four  and  five-tenths  microns 


44     THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 

in  diameter,  and  many  of  them  extend  from  the  dento-enamel 
junction  to  the  surface  of  the  enamel.  They  are  of  the  same 
diameter  at  their  outer  and  inner  ends.  This  last  statement 
is  emphasized,  as  the  direct  opposite  is  stated  in  some  stand- 
ard text-books  of  histology.  In  the  formation  of  the  tissue 
they  are  arranged  so  that  the  expansions  in  adjoining  rods 
come  opposite  to  each  other,  and  do  not  interlock  with  the 
constrictions,  so  that  there  is  alternately  a  greater  and  a  less 
amount  of  cementing  substance  between  them. 

Fig.  10 


Enamel  rods  isolated  by  scraping.      (About  SOO  X) 


It  is  evident  that  the  outer  surface  of  the  enamel  is  much 
greater  than  the  surface  of  the  dentine  at  the  dento-enamel 
junction.     This  greater  area  is  obtained  in  two  ways: 

1.  The  rods  are  at  right  angles  to  the  dentine  at  the 
dento-enamel  junction,  but  are  seldom  at  right  angles  to  the 
outer  surface.    This  may  be  illustrated  by  bending  the  leaves 


ENAMEL  RODS 


45 


of  a  book,  or  cutting  a  stack  of  paper  obliquely.  The  sheets 
of  paper  are  of  the  same  thickness,  but  when  cut  at  right 
angles  to  the  sheets  the  area  of  the  cut  surface  is  not  so 
great  as  when  the  leaves  are  cut  diagonally. 

2.  Many  of  the  enamel  rods  undoubtedly  extend  from  the 
dento-enamel  junction  to  the  surface  of  the  enamel,  though 
it  is  difficult  to  follow  individual  rods  through  this  distance, 


Fig.   11 


Enamel  rods  in  thin  etched  section.     (About  800  X) 


but  there  are  also  short  rods  which  extend  from  the  surface 
part  way  to  the  dentine.  These  short  rods  end  in  tapering 
points  between  converging  rods  that  extend  the  entire  dis- 
tance. The  short  rods  are  specially  numerous  in  the  most 
convex  portion  of  the  surface,  as  over  the  tips  of  the  cusps, 
occlusal  edges,  and  marginal  ridges.  These  areas,  therefore, 
become  of  special  importance  in  connection  with  the  forma- 


46      THE  STRUCTURAL  ELEMENTS  OF   THE  ENAMEL 

tion  of  enamel  walls,  as  will  be  considered  in  detail  later  on 
(Fig.  11). 

Differences  between  Enamel  Rods  and  Cementing  Substance. 
■ — AYliile  the  cementing  substance  and  the  substance  of  the 
rods  are  both  entirely  inorganic,  or,  more  correctly,  are  com- 
posed entirely  of  mineral  salts,  they  differ  in  physical  and 
chemical  properties  as  follows : 

1.  The  cementing  substance  is  not  as  strong  as  the  pris- 
matic substance. 

2.  The  cementing  substance  is  more  readily  soluble  in 
dilute  acids  than  the  rod  substance. 

3.  The  cementing  substance  is  of  slightly  different 
(greater)  refracting  index  than  the  substance  of  the  rod.  The 
author  wishes  to  emphasize  these  statements,  as  the  exact 
opposite  is  found  in  some  of  the  standard  texts,  at  least 
concerning  the  first  and  second  statements.  The  facts  are, 
however,  so  easily  demonstrable  that  anyone  may  satisfy 
himself  without  difficulty. 

Relative  Strength  of  the  Enamel  Rods  and  the  Cementing 
Substance. — The  cementing  substance  is  not  as  strong  as 
the  substance  of  the  rods.  The  most  striking  characteristics 
of  the  enamel,  and  the  first  to  attract  the  attention  of  the 
student  and  the  operator,  are  its  hardness  and  its  tendency 
to  split  or  cleave  in  certain  directions.  On  examination  it  is 
found  that  this  is  determined  by  the  direction  of  the  rods, 
and  is  caused  by  the  difference  in  strength  between  the  two 
substances.  Sections  ground  at  right  angles  to  the  rod 
direction  are  very  difficult  to  prepare  because  of  the  tendency 
of  the  section  to  break  to  pieces. 

If  a  section  that  is  beginning  to  crack  (Fig.  12)  is  studied, 
the  crack  is  found  to  follow  the  line  of  the  cementing  sub- 
stance running  around  the  rods.  In  some  places  a  rod  may 
be  split  through  its  centre,  but  most  of  the  rods  remain 
perfect,  and  the  cementing  substance  breaks.  In  the  same 
way  a  section  cut  in  the  direction  of  the  rods  shows  the  crack 
following  the  lines  of  the  cementing  substance  (Fig.  13), 
here  and  there  breaking  across  a  few  rods,  and  then  fol- 
lowing the  direction  again;  but   the  rods  separate  on  the 


RELATIVE  STRENGTH  OF  THE  ENAMEL  RODS      47 

line  of  union,  not  at  the  centres  of  the  rods.  This  fact 
becomes  fundamental  in  the  cutting  of  enamel  and  in  the 
preparation  of  strong  enamel  walls. 

Fig.  12 


Transverse  section  of  enamel  rods.      (.About  bO  X) 
Fig.  13 


Enamel  showing  direction  of  cleavage.      (About  70  X) 


48     THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 

Relative  Solubility  of  Enamel  Rods  and  Cementing  Sub- 
stance.— If  a  thin  section  of  enamel  cut  parallel  witli  the 
direction  of  the  enamel  rods  is  mounted  in  water  and  hydro- 
chloric acid  (2  per  cent.)  is  allowed  to  run  under  the  cover- 
glass  and  the  action  observed,  it  will  be  seen  to  attack  the 
cementing  substance  more  rapidly,  dissolving  it  out  from 
between  the  enamel  rods  and  attacking  their  sides.  If  the 
action  is  stopped  the  ends  of  the  rods  will  be  seen  pro- 
jecting like  the  pickets  of  a  fence,  as  shown  in  the  photograph 
(Fig.  14).  The  more  dilute  the  acid  the  greater  will  be  the 
distance  to  which  the  cementing  substance  is  removed  before 
the  rods  are  destroyed. 

Fig.   14 


The  effect  of  acid  on  a  section  of  enamel. 


Etching. — If  a  section  of  enamel  is  ground  at  right  angles 
to  the  direction  of  the  rods,  mounted  in  glycerin  and  photo- 
graphed, the  outline  of  the  rods  will  be  seen  with  difficulty 
(Fig.  15).  The  refracting  index  of  the  two  substances  is  so 
nearly  the  same  that  the  section  seems  of  almost  uniform 
transparency.  The  thinner  the  section,  the  greater  will  be 
the  difficulty  of  recognizing  the  rods.  Oblique  illumina- 
tion and  the  use  of  a  small  diaphragm  will,  however,  resolve 
them.  If  the  section  is  washed  and  treated  with  2  per  cent, 
hydrochloric  acid  for  a  few  seconds,  washed,  and  remounted 
in  glycerin,  the  rods  are  distinctly  outlined  (Fig.  16).  The 
acid  attacks  the  cementing  substance  and  the  surface  of 
the  section  is  etched  as  if  an  engraving  tool  had  been  run 


RELATIVE  SOLUBILITY  OF   THE  EX  AM  EL  RODS     49 

around  the  rods.  The  fine  grooves  on  the  surface  refract 
the  fight  and  outfine  the  rods.  The  difference  in  appear- 
ance in  longitudinal  sections,  that  is,  sections  parallel  with  the 
direction  of  enamel  rods,  is  quite  as  striking.  For  the  study 
of  enamel  rod  directions  this  etching  is  of  the  greatest  impor- 
tance.    Only  one  side  of  the  section  should  be  acted  upon 


Enamel  ground  at  right  angles  to  the  rods.       Not  treated  with  acid.      (About  500  X) 


by  the  acid,  and  the  section  should  be  mounted  etched  side 
up.  If  etched  upon  both  surfaces,  the  grooves  in  the  lower 
surface  cannot  be  in  focus  at  the  same  time  as  those  of  the 
upper  surface  and  will  blur  the  definition. 

The  difference  in  the  solubility  of  the  rods  and  cementing 
substance  is  beautifully  illustrated  in  the  effect  of  caries 
4 


50      THE  STRUCTURAL  ELEMENTS  OF   THE  ENAMEL 

on  the  structure  of  the  enamel  (see  ilhistrations  in  Chai)ter 
XII),  and  caries  of  the  enamel  cannot  be  understood  unless 
these  fundamental  facts  are  remembered.  The  question, 
"AVhat  causes  the  difference  in  solubility  between  the  enamel 
rods  and  the  cementing  substance?"  cannot  be  satisfactorily 
answered  at  the  i)rcsent  time.    While  both  the  rods  and  the 

Fig.   10 


The  same  section  as  Fig.  15  after  treatment  with  acid.      (About  500  X) 


cementing  substance  are  normally  composed  entirely  of 
inorganic  salts,  there  may  be  different  salts  in  the  two  sub- 
stances, or  the  salts  may  be  in  different  physical  condition. 
There  is  great  need  for  careful  work  in  this  field.  Recent 
work  has  strongly  emphasized  the  distinctness  of  the  two 
structural  elements  of  the  enamel. 


DIFFERENCE  IN  REFRACTING  INDEX  51 

First,  the  study  of  the  beginnings  of  caries  of  the  enamel, 
and  the  effect  of  caries  upon  the  structure  of  the  enamel, 
brought  out  the  difference  in  solubility  in  acids  and  showed 
the  extent  of  tissue  injury  before  a  cavity  is  formed.  Later, 
the  study  of  atrophy  developed  the  fact  that  certain  patho- 
logic or  abnormal  conditions  may  hinder  or  entirely  pre- 
vent the  formation  of  the  rods  while  the  cementing  sub- 
stance is  formed,  and  still  more  recently  the  investigation  of 
dystrophies  of  the  enamel  occurring  in  certain  prescribed 
localities,  showed  perfect  rod  formation  and  entire  absence 
of  the  cementing  substance.  These  facts  suggest  the  hypoth- 
esis that  the  enamel  rods  and  the  cementing  substance  have 
a  different  origin,  or  are  formed  by  different  cells,  and  that 
pathological  conditions  may  prevent  the  formation  of  one 
and  not  the  other.  In  view  of  these  factors  it  is  very  neces- 
sary that  a  new  investigation  of  the  process  of  enamel  for- 
mation be  undertaken,  as  present  knowledge  of  the  process 
does  not  explain  such  conditions. 

Difference  in  Refracting  Index  between  the  Rods  and  the 
Cementing  Substance. — The  cementing  substance  is  of  slightly 
greater  refracting  index  than  the  substance  of  the  rods.  If 
it  were  not  for  this  it  would  be  impossible  to  see  the  rods  in 
unetched  sections,  either  longitudinal  or  transverse.  The 
appearance  of  striation  seen  in  longitudinal  sections  is  also 
dependent  upon  this  difference  in  action  on  transmitted 
light. 


CHAPTER  V 

CHARACTERISTICS   OF  THE   ENAMEL   TISSUE 

From  what  has  been  said  of  the  structural  elements  of 
the  tissue,  their  physical  and  chemical  properties,  and  their 
arrangement  in  the  tissue,  it  is  apparent  that  the  striking 
characteristics  of  the  enamel  are  the  result  of  these  factors; 
and  that  it  can  be  intelligently  dealt  with  only  by  thinking  of 
it  always  in  these  terms. 


Enamel  showing  cleavage. 


The  enamel  may  be  crudely  compared  to  a  pavement  made 
up  of  tall  columns  closely  cemented  together  by  an  inorganic 
cement.  The  wear  comes  on  the  ends  of  the  columns,  and 
they  furnish  great  resistance  to  the  abrasion  of  friction. 


STRAIGHT  ENAMEL 


53 


When  supported  upon  a  good  and  elastic  foundation  it  is  very 
difficult  to  break  it  down,  but  when  an  opening  has  been 
made  in  it,  and  the  foundation  removed  from  underneath, 
the  columns  are  comparatively  easily  split  off  and  tumbled 
into  the  opening  (Fig.  17).  This  figure  is  crude,  but  it  is  a 
very  helpful  one  in  learning  to  think  of  the  enamel  in  terms 
of  its  structural  elements. 


Fig.  18 


Straight  enamel  rods.      (About  80  X) 


Straight  Enamel. — Upon  the  axial  surfaces  of  the  teeth  the 
rods  are  usually  straight  and  parallel  with  each  other,  and 
most  of  them  extend  from  the  dentine  to  the  surface.  Such 
enamel  will  split  or  cleave  in  the  direction  of  the  rods  with 


54      CHARACTERISTICS  OF   THE  ENAMEL   TISSUE 

comparative  ease,  and  breaks  down  very  readily  when  the 
dentine  is  removed  from  under  it.  It  will  usually  cleave 
through  its  entire  thickness  and  break  away  from  sound 
dentine  when  properly  attacked  with  sharp  hand  instru- 
ments. Such  enamel  is  called  straight  enamel,  as  con- 
trasted with  gnarled  enamel.  It  is  best  illustrated  by 
cutting  sections  lal)iolingualIy  through  the  incisors,  though 
there  is  consi(leral)le  variation  in  different  teeth  (Figs.  13 
and  18). 

Fig.  19 


Gnarled  enamel.      (About  80  >0 


Gnarled  Enamel. — Upon  the  occlusal  surfaces  of  the 
molars  and  bicuspids,  and  especially  over  the  tips  of  cusps 
and   marginal   ridges,   the   rods   are   seldom   straight   and 


GNARLED  ENAMEL 


55 


parallel  through  the  thickness  of  the  enamel,  but  are  wound 
and  twisted  about  each  other,  especially  in  the  deeper  half 
toward  the  dento-enamel  junction.  This  is  known  as 
gnarled  enamel,  and  its  appearance  is  in  marked  contrast 
with  straight  enamel. 

Fig.  20 


Gnarled  enamel.      (About  50  X) 


Toward  the  surface  the  rods  are  usually  straight  and 
parallel  for  a  longer  or  shorter  distance,  but  as  the  dento- 
enamel  junction  is  approached  they  become  twisted.  This 
is  true  of  most  of  the  occlusal  surfaces  of  molars  and  bicus- 
pids, but  the  gnarled  condition  extends  farther  toward  the 
surface-over  the  tips  of  the  cusps,  or  the  point  at  which  the 
rods  were  first  completed  in  the  growth  of  the  crown.    As 


5G       CHARACTERISTICS  OF  THE  ENAMEL  TISSUE 

cleavage  is  caused  by  the  difference  in  the  strength  of  the 
rods  and  cementing  substance,  it  is  easy  to  see  that  gnarled 
enamel  will  not  split  or  cleave  easily  when  resting  upon 
sound  dentine.  This  is  often  encountered  in  extending 
occlusal  cavities.  The  straight  portion  will  split,  but  where 
the  rods  begin  to  twist  they  break  off,  leaving  a  portion 
resting  on  the  dentine  w-hich  will  resist  the  attack  of  any 
cutting  instrument  from  the  surface  (Figs.  19,  20,  and  21). 

Fig,  21 


Gnarled  enamel  from  etched  section.     (About  100  X) 


The  Effect  of  Structure  on  the  Cutting  of  Enamel. — The  two 

kinds  of  enamel  may  be  compared  to  straight-grained  pine 
wood  and  a  pine  knot.  The  first  will  split  easily  in  the 
direction  of  the  fibers,  the  latter  will  split  only  in  an  irregu- 
lar way  and  with  the  greatest  difficulty.  This  difference  in 
the  arrangements  of  the  structural  elements  leads  to  the 
difference  in  the  feeling  of  various  teeth  to  cutting  instru- 
ments, and  is  the  basis  for  the  clinical  experience  of  hard 
and  soft  teeth.    It  is  not  a  matter  of  degree  of  calcification, 


APPEARANCES  CHARACTERISTIC  OF  ENAMEL      57 

but  the  arrangement  of  the  structural  elements,  and  gnarled 
enamel  will  break  down  as  rapidly  under  the  effect  of  caries 
as  straight  enamel  would. 

From  a  study  of  the  positions  in  which  the  rods  are 
usually  twisted  about  each  other,  and  those  in  which  they 
are  usually  straight,  it  seems  probable  that  the  twisting  is 
due  to  movements  in  the  dental  papilla  and  the  enamel 
organ  during  the  formation  of  the  tissue.  These  movements 
may  be  produced  by  variations  in  the  blood  pressure  which 
cause  oscillations,  or  shifting  of  the  tissues  on  each  other. 
These  differences  in  the  arrangement  of  the  structural 
elements  of  the  enamel  must  be  constantly  kept  in  mind, 
and  will  be  referred  to  many  times  in  connection  with  the 
use  of  cutting  instruments  on  the  enamel  and  the  preparation 
of  cavity  walls. 

APPEARANCES  CHARACTERISTIC  OF  ENAMEL 

Striation. — Striation  is  the  appearance  of  fine  light  and 
dark  markings  occurring  alternately  in  the  length  of  the 
enamel  rods.  This  is  not  unlike  the  striation  of  voluntary 
muscle  fibers,  and  has  a  similar  cause.  It  is  seen  both  in 
thin  sections  cut  in  the  direction  of  the  rods,  and  in  isolated 
enamel  rods.  It  is  caused  by  the  alternate  expansions  and 
constrictions  of  the  rods  and  the  difference  in  the  refracting 
index  between  the  rods  and  the  cementing  substance. 

If  isolated  rods  (Fig.  22)  are  observed  with  a  -g^  or  yV  objec- 
tive, they  will  be  seen  to  be  marked  by  alternate  light  and 
dark  areas  across  the  rods;  on  changing  the  focus  up  and 
down,  the  light  and  dark  areas  will  change  places,  just  as  in 
looking  at  a  red  blood  corpuscle  the  centre  may  appear  dark 
and  the  rim  light,  or  the  centre  light  and  the  rim  dark, 
depending  upon  the  exactness  of  focus.  This  is  caused  by  the 
refraction  of  the  light  as  it  passes  through  the  convex  and 
concave  portions  of  the  rod.  If  the  cementing  substance  were 
of  exactly  the  same  refracting  index  as  the  rods,  when  the 
rods  were  fastened  together  in  the  tissue  there  would  be  no 
appearajice  of  striation,  but  as  it  is  not,  refraction  of  light 


58       CHARACTERISTICS  OF  THE  ENAMEL  TISSUE 

occurs  in  ])assiiig  from  rod  substance  to  cementing  sub- 
stance, and  the  striation  is  apparent  in  sections.  There  is 
considerable  difference  in  the  distinctness  of  striation  in 
different  sections  of  enamel.  This  is  probably  due  to  the  fact 
that  the  cementing  substance  has  more  nearly  the  same 
refracting  index  as  the  rods  in  some  specimens.  When  the 
formation  of  enamel  has  been  studied  it  will  be  found  that 
the  enamel  rods  have  been  formed  by  globules  which  are 

Fig    22 


^    3^\ 


Isolated  enaniL'i  r')d^<.     iAiMun    1000  X.) 

deposited  one  on  top  of  the  other  to  form  the  rods,  and  the 
cementing  substance  fills  up  the  space.  The  globules  in 
the  adjacent  rods  come  opposite  each  other,  so  that  there 
is  alternately  a  greater  and  a  less  amount  of  cementing 
substance  between  the  rods.  Each  cross-mark,  therefore, 
represents  a  globule  deposited  in  the  formation  of  the  rod, 
and  striation  may  be  said  to  be  a  record  of  the  growth  of  the 
individual  rods  (Figs.  23  and  24). 


APPEARANCES  CHARACTERISTIC  OF  ENAMEL     59 


Fig.  23 


1 

1 

■ 

■ 

■ 

i 

B 

? 

^B 

^M 

H 

■ 

"-■■ 

JH 

^^H 

w^ 

^^ 

'^H 

m^Hi 

^K^ 

m  - 

^ 

H^l 

iv 

.   ;.'  ' 

flrv'  r 

m 

B 

U      ';--> 

rJH 

^Ktt_ 

-:        .v^ 

E&nT'V 

i 

hm 

■ 

H^ 

«■ 

■ 

^^B 

H| 

1 

— ^^ 

^•,-"    .'    '"-  •.  •-; 

/ .^ 

■ 

,-. ,. 

^^^i 

MB 

■ 

IKHI 

IMH 

IHIH 

Knamei  showing  both  striation  and  stratification.      (About  SO  X) 
Fig.  24 


Enamel  showing  striation.      (About  1000  X) 


60      CHARACTERISTICS  OF  THE  ENAMEL  TISSUE 

Imperfections  in  the  cementing  substance  render  the 
striation  more  apparent  because  they  increase  the  difference 
in  refraction  between  the  two  substances.  The  action  of 
acid  either  upon  isolated  rods  or  upon  sections  renders 
striation  more  apparent  because  it  attacks  the  cementing 
substance  faster  than  the  globules  forming  the  rods,  and 
therefore  increases  the  refraction.  Von  Beber  has  claimed 
that  the  appearance  of  striation  was  caused  by  the  action  of 
acid  on  the  section,  and  that  even  in  mounting  in  balsam 
the  acidity  of  the  balsam  affected  the  tissue.  It  is  true  that 
any  action  of  acid  increases  the  distinctness  of  the  cross- 
striation,  but  it  is  not  the  cause  of  it. 

Stratification,  or  the  Bands  of  Retzius. — If  longitudinal 
sections  of  moderate  thickness  are  observed  with  the  low 
power,  brownish  bands  are  seen  running  through  the  enamel, 
which  suggests  the  appearance  of  stratification  in  rocks. 
These  were  first  described  by  Retzius  and  were  named  after 
him — the  brown  bands  or  striae  of  Retzius.  A  better  name 
would  be  incremental  lines. 

The  bands  of  Retzius,  or  incremental  lines,  are  caused  by 
actual  coloring  matter  which  is  deposited  with  the  inorganic 
salts  in  the  formation  of  the  tissue.  They  are,  therefore, 
best  seen  with  low  powers  and  in  sections  that  are  not  too 
thin.  In  sections  that  are  thinner  than  the  diameter  of  a 
single  rod,  or  less  than  four  microns,  they  become  almost 
invisible.  For  the  study  of  the  bands  of  Retzius  sections 
should  be  ground  labiolingually  through  the  incisors,  bucco- 
lingually  through  the  bicuspids  and  molars,  striking  the 
centre  at  the  cusps.  They  may  be  studied  also  in  mesiodistal 
sections,  but  the  sections  should  be  in  such  a  direction  as  to 
be  at  right  angles  to  the  zones.  Fig.  25  shows  the  tip  of  an 
incisor  in  which  the  bands  are  very  well  marked.  They  are 
seen  to  begin  at  the  dento-enamel  junction  on  the  incisal 
edge,  and  sweep  in  larger  and  larger  zones  around  this 
point.  Each  band  represents  what  was  at  one  time  the  sur- 
face of  the  enamel  already  formed,  and  the  line  upon  which 
formation  was  progressing.  They  are,  therefore,  truly 
incremental  lines.      The  zones  reach  the    surface    of  the 


APPEARANCES  CHARACTERISTIC  OF  ENAMEL     61 

enamel  first  at  the  point  over  the  centre  of  beginning  calcifi- 
cation, and  the  succeeding  bands  extend  from  the  surface 

Fig.  25 


Tip  of   an  incisor.      (About  50  X) 


of  the  enamel,  near  the  occlusal,  to  the  dento-enamel  junction 
much  farther  apically,  and  corresponding  lines  are  seen  on 
opposite  sides  of  the  section.     In  Fig.  26  the  band  which 


C)2      CHARACTERISTICS  OF  THE  ENAMEL  TISSUE 

is  at  the  surface  at  A  and /I'  reaches  the  dento-enamel  junc- 
tion at  B  and  B\  This  means  that  when  the  enamel  rods 
which  form  the  surface  at  A  were  completed,  the  rods  at 

Fig.  26 


Incisor  tip  showing  stratification  or  incremental  lines.      Rods  at  A  were  fully  formed 
at  the  time  the  rods  at  B  were  Vjeginning  to  form.      (About  50  X) 


B  were  just  beginning  to  be  formed  at  the  dento-enamel 
junction.  A  layer  of  functioning  ameloblasts  occupied 
this   position.      The  bands  of   Retzius  are  always  curved 


APPEARANCES  CHARACTERISTIC  OF  ENAMEL     63 

and  usually  pass  obliquely  across  the  enamel  rods,  but 
are  parallel  neither  with  the  dento-enamel  junction  or  the 
surface  of  the  enamel.     As  they  pass  toward  the  gingival 

Fig.  27 


Stratification  of  enamel:  the  cusp  of  a  bicuspid:  De.  dento-enamel  junction:  Ed, 
enamel  defecj  showing  in  the  heavj'  stratification  band;  Ig,  interglobular  spaces  in 
the  dentine.      (About  40  X) 


64      CHARACTERISTICS  OF  THE  ENAMEL  TISSUE 

the  angle  which  they  form  with  the  axis  of  the  tooth  becomes 
greater.  Any  disturbance  of  nutrition  which  affects  the 
formation  of  enamel  is  always  shown  in  the  increased  distinct- 
ness of  the  bands  (Fig.  27). 

The  bands  of  Retzius,  therefore,  form  a  record  of  the 
formation  of  the  tissue,  and  by  their  study  the  points  of 
beginning  calcification  and  the  manner  of  the  development 
of  the  tooth  crown  may  be  followed.  This  will  be  con- 
sidered again  in  connection  with  the  grooves,  pits,  and 
natural  defects  of  the  enamel. 

Fig.  28 


^ 


Lines  of  Schreger.      (About  5  X) 

Lines  of  Schreger. — These  are  lines  appearing  in  the  enamel 
extending  from  the  dento-enamel  junction  to  or  toward  the 
surface.  They  are  caused  by  the  direction  in  which  the 
enamel  rods  are  cut.  They  may  be  seen  in  sections,  but  are 
best  shown  by  photographing  the  cut  surface  of  the  enamel 
by  reflected  light  and  with  very  low  magnification.  The 
rods  are  twisting  about  each  other,  and  in  one  streak  they 
are  cut  longitudinally,  in  the  next  obliquely,  and  the 
alternations  of  these  directions  cause  the  appearance  of 
the  lines  (Fig.  28). 


CHAPTER  VI 

THE   DIRECTION   OF  THE  ENAMEL  RODS   IN  THE 
TOOTH  CROWN 

In  describing  the  direction  of  the  enamel  rods  and  their 
arrangement  in  what  may  be  called  the  architecture  of  the 
tooth  crown,  they  are  always  considered  as  extending  from 
the  dento-enamel  junction  outward.  This  is  not  only  con- 
venient, but  logical,  as  they  are  formed  in  that  way,  beginning 
at  the  dento-enamel  junction  and  being  completed  at  the 
surface.  Enamel  is  formed  from  within  outward,  the  cells 
which  produce  it  lying  outside  of  the  tissue  already  formed, 
and  there  are  many  things  about  the  arrangement  of  the  rods 
and  their  relation  to  each  other  that  are  understood  only 
when  this  is  borne  in  mind. 

The  direction  of  the  enamel  rods  is  described  by  referring 
them  to  the  horizontal  and  axial  planes,  which  have  been 
previously  defined  (page  37).  The  centigrade  scale,  that 
is  the  division  of  the  circle  into  one  hundred  equal  arcs,  is 
used  because  those  familiar  with  instrument  nomenclature 
are  already  familiar  with  these  angles,  and  readily  picture 
them.i     When  a  rod  is  said  to  be  inclined  12  centigrades 


1  In  the  centigrade  division 
the  circle  is  divided  into  one 
hundred  parts,  each  called  a 
centigrade.  One  centigrade  is 
equal  to  3.6  degrees  of  the 
astronomical  circle.  25  centi- 
grades to  90  degrees,  12f  cen- 
tigrades to  45  degrees.  The 
cut  gives  a  comparison  of  the 
two  systems  of  measuring 
angles 


2  70 


180'' 
Centigrade  di\'ision. 


GO     DIRECTION  OF  ENAMEL  RODS  IN   TOOTH  CROWN 

occlusally  from  tlie  horizontal  plane,  it  means  that  if  a 
plane  at  right  angles  to  the  long  axis  of  the  tooth  is  passed 
through  the  end  of  the  rod  at  the  dento-enamel  junction,  the 
rod  will  lie  to  the  occlusal  of  it  and  form  an  angle  of  12  cen- 
tigrades  with  it.  In  the  same  way,  if  a  rod  is  said  to  be 
inclined  12  centigrades  buccally  from  the  mesiodistal  plane, 
it  means  that  if  a  plane  parallel  with  the  axis  of  the  tooth, 
and  extending  from  mesio  to  distal,  is  passed  through  the 
end  of  a  rod  at  the  dento-enamel  junction,  the  rod  will  lie  to 
the  buccal  of  it,  and  form  an  angle  of  12  centigrades  with  it. 
By  a  little  practice  with  these  terms  the  direction  of  the 
enamel  rods  can  be  very  easily  and  clearly  pictured  to  the 
mind. 

The  General  Direction  of  Enamel  Rods. — The  general  direc- 
tion of  the  enamel  rods  has  been  variously  described  by 
different  authors,  but  all  of  these  general  statements  are 
very  imperfect  and  often  misleading.  For  instance,  they 
are  sometimes  said  to  radiate  from  the  centre  of  the  crown 
or  the  pulp  chamber,  but  it  will  be  seen  that  this  does  not 
apply  to  the  rods  which  form  the  lingual  slopes  of  the  buccal 
cusps,  or  the  buccal  slopes  of  the  lingual  cusps  of  bicuspids 
and  molars. 

Again,  they  have  been  said  to  be,  in  general,  perpendicular 
to  the  surface,  but  it  will  be  found  from  the  study  of  sections 
that  there  are  very  few  places  upon  the  surface  where  this  is 
true,  and  that  in  many  places  they  are  far  from  perpendicu- 
lar to  the  surface.  From  a  stud}^  of  sections  it  wdll  be  seen 
that  the  general  arrangement  of  enamel  rods,  in  the  archi- 
tecture of  the  tooth  crown  is  such  as  to  give  the  greatest 
strength  to  the  perfect  tissue,  and  to  furnish  the  greatest  resist- 
ance to  abrasion  in  the  use  of  the  teeth  for  mastication.  In  a 
buccolingual  section  through  a  bicuspid  (Fig.  29),  beginning 
at  the  gingival  line,  the  enamel  is  normally  slightly  overlapped 
by  the  cementum,  and  in  the  gingival  third  the  rods  are 
inclined  more  or  less  apically  from  the  horizontal  plane. 
The  degree  of  inclination  varies  considerably.  It  may  be 
as  much  as  12  centigrades,  but  is  usually  not  more  than  6. 
In  general,  the  more  convex  the  surface  the  greater  will 


THE  GENERAL  DIRECTION  OF  ENAMEL  RODS     67 

be  the  inclination.  At  some  point  between  the  junction  of 
the  gingival  and  middle  thirds  and  the  middle  of  the  middle 
third  of  the  surface  they  are  in  the  horizontal  plane  and  at 
right  angles  to  the  axis  of  the  tooth,  and  at  this  point  they 
are  usually  very  nearly  perpendicular  to  the  surface.     Passing 


Fig.  29 


Diagram  of   enamel   rod    directions,  from  a  photograph  of   a  buccolingual  section  of 
an  upper  bicuspid. 


occlusally  from  this  point,  they  incline  more  and  more 
occlusally  until  in  the  occlusal  third  they  reach  an  inclina- 
tion of  18  to  20  centigrades  occlusalh'  from  the  horizontal. 

The  rods  which  form  the  tip  of  the  buccal  cusps  do  not 
reach  the  tip  of  the  dentine  cusp,  but  the  buccal  slope  of 


68     DIRECTION  OF  ENAMEL  RODS  IN  TOOTH  CROWN 

the  dentine.  This  becomes  important,  as  will  be  seen  later. 
Over  the  tip  of  the  dentine  cusp  the  rods  are  in  the  axial 
plane,  but  in  this  position  they  are  usually  very  much 
twisted.  Passing  down  the  lingual  slope,  they  become  more 
and  more  inclined  lingually  from  the  mesiodistal  axial  plane. 

Fig.  30 


Diagram  of  enamel  rod  directions,  drawn  from  a  mesiodistal  section  of  a  bicuspid. 

and  the  degree  of  inclination  is  related  to  the  height  of 
the  cusp — the  taller  the  cusp  the  greater  the  inclination.  At 
the  developmental  groove  or  pit  they  meet  the  rods  of  the 
lingual  cusp,  which  are  inclined  in  the  opposite  direction. 

In  a  mesiodistal  section  (Fig.  30)  the  plan  of  arrangement 
will  be  seen  to  be  the  same,  the  tip  of  the  marginal  ridge 
corresponding  to  the  tip  of  the  cusp.    In  an  incisor  the 


THE  GENERAL  DIRECTION  OF  ENAMEL  RODS      69 

I 
Fig.  31  1 


Disturbance  of  enamel  rod  directions  on  labial  surface  of  a  cuspid.       (About  SO  X 


70     DIRECTION    OF  ENAMEL    RODS  IN   TOOTH  CROWN 


Fia.  32 


Disturbance  of  enamel  rod  directions  on  lingual  surface  of  same  tooth  as  Fig.  33. 
(About  80  X) 


SPECIAL  AREAS  71 

arrangement  is  similar,  the  lingual  marginal  ridge  corre- 
sponding to  a  rudimentary  cusp.  This  general  plan  should 
be  studied  in  several  sections  of  the  various  classes  of  teeth 
before  the  rod  direction  is  studied  more  minutely. 

Effect  of  Atrophy. — Whenever  an  atrophy  groove  appears 
upon  the  surface,  the  rod  directions  will  be  found  to  be  more 
or  less  disturbed.  Fig.  31  shows  a  position  on  the  labial 
surface  of  a  cuspid.  In  this  position  the  disturbance  of  the 
enamel  rod  direction  is  very  marked.  The  rods  tend  to  be 
in  whirls  and  the  structure  is  more  or  less  deficient.  On  the 
lingual  side  of  the  same  section  (Fig.  32)  the  disturbance  in 
structure  is  so  great  that  it  is  difficult  to  make  out  the  rod 
direction.  Many  such  areas  will  be  found  in  sections.  Some 
condition  which  has  affected  the  nutrition  of  the  enamel- 
forming  cells  results  in  a  local  disturbance  of  the  structural 
elements. 

SPECIAL  AREAS 

The  Gingival  Third. — There  is  much  variation  in  enamel 
rod  direction  in  difierent  teeth  as  the  gingival  line  is 
approached.  The  inclination  apically  from  the  horizontal 
may  be  very  great,  as  much  as  12  to  15  centigrades  in 
some  instances,  as  in  Fig.  33,  but  this  is  exceptional. 
It  may  be  very  slight,  or  the  rods  may  be  almost  in  the 
horizontal  plane.  The  direction  of  the  rods  in  these  areas 
become  very  important  in  the  preparation  of  the  gingival 
wall  of  proximal  cavities,  and  cavities  in  the  gingival  third 
of  buccal  and  labial  surfaces. 

The  Tips  of  the  Cusps. — In  studying  the  rod  directions 
in  the  region  of  the  cusps  and  marginal  ridges,  it  must  be 
borne  in  mind  that  the  formation  of  enamel  begins  at  the 
dento-enamel  junction,  at  separate  points,  and  that  the 
growth  is  recorded  in  the  tissue  by  the  bands  of  Retzius, 
each  band  having  been  at  one  time  the  surface  of  the  enamel 
cap  then  formed.  In  a  buccolingual  section  the  formation 
of  the  buccal  and  lingual  cusps  will  be  shown  (Chapter  X). 
While  the  little  caps  are  growing  they  are  being  carried 
apart  by  the  growth  of  the  dental  papilla  and  enamel 
organ,  until   the   calcifications   unite  at  the  dento-enamel 


72     DIRECTION  OF  ENAMEL  RODS  IN  TOOTH  CROWN 

junction.  When  this  occurs  the  dental  papilla  has  reached 
its  maximum  mesio-distal  diameter.  The  enamel  organ, 
however,  will  continue  to  grow,  and  as  the  rods  are  com- 

FiG.  33 


Direction  of  enamel  rods  in  the  gingival  third. 


pleted  first  just  over  the  tip  of  the  dentine  cusp,  the  con- 
tinued growth  causes  an  increase  in  the  inclination  of  the 
rods  in  their  outer  portion.  This  often  leads  to  a  curving 
of  the  rods  at  their  outer  portion. 


CHAPTER  VII 

THE  RELATION  OF  THE  STRUCTURE  TO  THE  CUTTING 
OF  THE  ENAMEL 

•  There  are  two  methods  of  cutting  enamel — to  chop  or 
cleave  it,  or  to  shave  or  plane  it. 

Cleaving  or  Chopping  Enamel. — In  the  cleavage  of  the 
enamel  the  action  of  the  instrument  more  nearly  resembles 
that  of  splitting  ice  than  that  of  splitting  wood.  The  ax  for 
splitting  wood  is  strongly  wedge-shaped,  and  the  wedge 
pries  the  fibers  apart.  In  splitting  ice  a  small  nick  is  made 
on  the  surface  and  then  a  sharp  blow  cracks  the  ice  in  the 
direction  of  the  cleavage.  In  a  similar  way  the  chisel 
applied  to  the  surface  of  the  enamel  makes  a  slight  scratch 
or  bearing  on  the  surface,  and  the  force  applied  at  a  slight 
angle  to  the  direction  of  the  rods  cracks  the  tissue  through 
in  the  rod  direction.  The  bevel  of  the  instrument  is 
designed  to  give  strength  and  keenness  of  edge,  not  to  act 
as  a  wedge.  In  order  to  cleave  the  enamel  it  is  always 
necessary  that  there  be  a  break  or  opening  in  the  tissue. 
Only  a  small  portion  can  be  split  off  at  a  time.  The  edge 
of  the  chisel  should  be  placed  on  the  enamel  a  quarter  or 
half  a  millimeter  from  the  opening,  rarely  more,  and  so 
piece  after  piece  is  split  into  the  cavity.  Fig.  34  shows  a 
section  of  enamel.  The  edge  of  the  chisel  is  placed  at  1,  with 
the  shaft  in  the  relation  to  enamel  rod  direction  indicated; 
a  tap  of  a  steel  mallet  will  split  oft'  a  piece,  and  the  chisel  is 
moved  back  to  position  2  and  a  second  piece  is  split  off. 
Undermined  enamel  will  split  easily  in  this  way.  As  soon  as 
a  point  is  reached  where  the  enamel  rests  on  sound  dentine, 
it  is  recognized  by  the  resistance.  Straight  enamel  can  be 
split  off  from  sound  dentine  without  difficulty  if  attacked 
in  the  proper  way,  but  if  the  inner  portion  is  gnarled  and 


74     RE  LA  TION  OF  S  TR  UC  T  URE  TO  CUT  TING  OF  EN  A  MEL 

twisted,  it  can  only  be  cleaved  by  removing  the  dentine 
from  under  it.    Such  enamel,  if  resting  on  dentine,  will  split 


Position  of  chisel  in  clea\ 


as  far  as  the  rods  are  straight;  but  where  they  begin  to  twist 
they  will  break  off,  leaving  a  portion  which  is  very  difficult 
to  remove  by  attacking  it  from  the  surface.    If  the  dentine 


CLEAVING  OR  CHOPPING  ENAMEL  75 

is  removed  from  under  gnarled  enamel,  it  will  crack  through 
in  an  irregular  way,  following  the  general  direction  of  the 
rods. 

In  preparing  teeth  for  crowns  it  is  often  necessary  to 
remove  a  large  amount  of  enamel.  This  is  always  more 
efficiently  accomplished  by  the  intelligent  use  of  sharp 
instruments  than  by  force.  The  enamel  on  axial  surfaces, 
especially  in  the  gingival  half  of  the  crown,  is  usually 
straight,  and  if  a  cleavage  line  can  once  be  established, 
the  enamel  can  be  more  easily  and  rapidly  removed  by 
splitting  it  off  piece  after  piece  than  in  any  other  way. 
In  doing  this  a  straight  or  contra-angled  chisel  is  often  the 
most  efficient  instrument,  and  it  must  be  remembered  that 
the  "root  trimmers"  are  more  properly  called  "enamel 
cleavers,"  and  that  they  are  used  to  cleave  the  enamel, 
not  to  scrape  or  hoe  it  off,  their  form  being  adapted  to  give 
a  strong  palm  grasp  of  the  instrument. 

Fig.  35  illustrates  the  use  of  the  enamel  cleaver  for  the 
removal  of  gingival  enamel  from  an  axial  surface.  The  line 
of  cleavage  being  established,  the  edge  of  the  instrument  is 
placed  on  the  enamel  half  a  millimeter  from  the  broken  edge, 
and  the  force,  which  should  be  strong,  quick,  and  sharp,  is 
applied  in  the  direction  indicated,  and  piece  after  piece  is 
split  off,  progressing  from  the  occlusal  toward  the  gingival. 
In  preparing  the  wall  of  a  cavity  the  outline  form  should 
be  attained  by  cleavage,  and  this  is  the  first  step  in  the 
preparation  of  the  cavity. 

After  the  enamel  has  been  removed  by  cleavage  to  the 
point  where  the  margin  is  to  be  laid,  the  wall  must  be 
completed  by  cutting  the  enamel  in  an  entirely  different 
way. 

Planing  or  Shaving  Enamel. — In  this  manner  of  cutting 
enamel  the  tissue  is  removed  without  reference  to  the  rod 
direction,  and  without  injury  to  its  structure  (Figs.  36,  37, 
and  38).  The  chisel  is  used  like  the  blade  of  a  plane.  The 
cutting  edge  is  placed  against  the  surface  with  the  shaft  of 
the  instrument  almost  parallel  to  it,  and  the  tissue  is  shaved 
away.    In  this  way  the  rods  that  have  been  cracked  apart 


76     RELATION  OF  STRUCTURE  TO  CUTTING  OF  ENAMEL 

by  the  cleavage  are  removed,  and  the  walls  arranged  in 
terms  of  its  structural  elements  so  as  to  gain  the  required 
strength  of  margin. 

Fig.  35 


^— 


The  use  of   enamel  cleaver  in  removing  enamel. 

Sharp  Instruments. — Chisels  and  hatchets  for  use  in  cleaving 
or  planing  enamel  must  be  keenly  sharp.     If  a  dull  edge  is 


SHARP  INSTRUMENTS  77 

Fig.  36  Fig.  37  Fig.  38 


The  use  of  the  chisel  in  planing  or  shaving  enamel.      (Black.) 
Fig.  39 


The  relation  of  the  edge  of  a  sharp  and  a  dull  chisel. 


7S     RELA  TIOX  OF  STRUCTURE  TO  CUTTING  OF  ENAMEL 

])lacc(l  on  the  surface  of  the  enamel  it  will  rest  across  the 
ends  of  many  rods,  and  force  applied  will  only  crumble  them, 
but  will  not  split  the  tissue.  The  edge  must  be  keen  (Fig. 
.39),  so  as  to  engage  between  the  rods  and  so  start 
the  cleavage.  Cutting  instruments  as  furnished  by  dental 
supply  houses  are  not  tempered  hard  enough  to  hold  an 
edge.  There  is  no  fault  to  be  found  with  the  supply  houses 
for  this,  for  they  make  them  as  the  dentist  wants  them,  and 
any  dealer  will  furnish  hard-tempered  instruments  if  they 
are  ordered.  To  use  hand  instruments  successfully  in  cutting 
enamel,  the  stock  instruments  must  either  be  retempered  or 
the}'  must  be  ordered  hard  tempered.    The  cutting  edge  of 


Fig.  40 


Fig.  41 


Fig.  42 


The  use  of   the  chisel  in  cleaving  enamel.     Opening  an  occlusal  cavity.      (Black.) 

the  blade  of  an  enamel  instrument  should  be  straw-colored 
when  tempered.  The  chisel  and  hatchets  are  the  instruments 
for  removing  enamel.  The  burr  is  the  instrument  for 
removing  hard  dentine.  When  the  burr  is  used  on  enamel 
it  should  be  remembered  that  it  is  used  as  a  revolving 
chisel.  It  is  by  the  thoughtful  use  of  hand  instruments 
that  knowledge  of  enamel  rod  direction  is  gained,  and 
only  by  the  use  of  them  can  the  enamel  walls  be  pre- 
pared in  terms  of  their  structural  elements.  In  cleaving 
undermined  enamel  the  edge  may  be  used  either  with  a 
pulling  or  a  pushing  motion.  For  instance,  in  opening  up 
a  cavity  in  the  occlusal  surface  of  a  bicuspid,  the  buccal 


SHARP  INSTRUMENTS  79 

portion  of  undermined  enamel  is  split  off  by  placing  the 
instrument  as  shown  in  Figs.  40  and  41 .  The  bevel  of  the  blade 
is  held  toward  the  cavity  and  the  shaft  of  the  instrument  at 
a  slight  angle  to  the  rod  direction,  and  the  force  is  applied 
in  the  direction  of  the  shaft.  The  lingual  portion  may 
be  removed  by  placing  the  instrument  as  indicated  in  Fig. 
42,  the  bevel  of  the  blade  away  from  the  cavit}^  and  the 
force  applied  in  the  direction  of  the  bevel  by  a  pulling  force 
in  the  direction  of  the  shaft.  This  is  the  way  in  which  force 
is  applied  on  enamel  cleavers.  The  pitch  of  the  bevel  in  an 
enamel  cleaver  and  its  relation  to  the  shaft  of  the  instrument 
is  extremely  important,  and  the  efficiency  of  an  instrument 
may  easily  be  ruined  by  careless  honing.  Every  time  a 
cutting  instrument  is  applied  to  the  enamel  it  must  be 
done  with  a  knowledge  of  the  relation  of  the  cutting  edge 
and  the  force  to  the  direction  of  the  enamel  rods,  until  it 
becomes  entirely  automatic.  The  author  emphatically 
believes  that  the  acquirement  of  this  knowledge  and  skill 
will  do  more  to  increase  facility  and  success  in  the  prepara- 
tion of  cavity  walls  than  any  other  manipulative  factor. 
The  preparation  of  enamel  walls  requires  the  continual 
application  of  the  knowledge  of  enamel  structure.  Enamel  is 
a  very  hard  tissue,  but  it  is  composed  of  structural  elements, 
and  walls  prepared  without  reference  to  them  will  prove 
their  own  weakness. 


CHAPTER  VIII 

THE  STRUCTURAL  REQUIREMENTS  FOR  STRONG 
ENAMEL  WALLS 

From  the  consideration  of  the  physical  character  of  the 
enamel,  its  structural  elements  and  their  properties,  it  is 
evident  that  the  strength  of  any  enamel  wall  is  dependent 
upon  the  arrangement  of  the  rods  in  the  tissue  which  makes 
up  the  walls  and  their  relation  to  the  dentine.  Certain 
requirements  for  strength  can  be  clearly  stated,  and  these 
are  applicable  to  all  enamel  walls.  They  cannot  always  be 
secured  with  equal  facility  or  perfection,  but  in  proportion 
as  these  principles  are  observed  and  attained  the  wall  will 
be  strong;  as  they  are  imperfectly  attained  or  ignored  the 
wall  will  be  weak  and  unreliable.  When  these  conditions  are 
understood  very  many  failures  can  be  clearly  seen  to  have 
been  the  result  of  their  neglect. 

Structural  Requirements.^ — 1.  The  enamel  must  rest  upon 
sound  dentine. 

2.  The  rods  which  form  the  cavosurface  angle  must  have 
their  inner  ends  resting  upon  sound  dentine. 

3.  The  rods  which  form  the  cavosurface  angle  must  be 
supported  by  a  portion  of  enamel  in  which  the  inner  ends  of 
the  rods  rest  on  sound  dentine  and  the  outer  ends  are 
covered  by  the  filling  material. 

4.  The  cavosurface  angle^  must  be  so  trimmed  or  bevelled 
so  that  the  margin  will  not  be  exposed  to  injury  in  con- 
densing the  filling  material  against  it  (Fig.  43). 

These  requirements  should  be  considered  one  by  one. 
The  Enamel  Must  Rest  upon  Sound  Dentine. — That  is,  the 
enamel  plate  must  have  the  support  of  sound  dentine,  and 

1  The  cavosurface  angle  is  defined  as  the  angle  formed  by  the  surface  of  the  tooth 
and  the  wall  of  the  cavity. 


ENAMEL  MUST  REST  UPON  SOUND  DENTINE     81 

all  portions  which  are  undermined  by  the  removal  of  dentine 
must  be  cut  away.  When  the  inner  ends  of  the  rods  which 
form  the  enamel  plate  rest  upon  sound  dentine,  the  elasticity 
of  the  dentine  gives  to  the  enamel  a  certain  degree  of  elas- 
ticity^, but  the  enamel  itself  without  this  support  is  extremely 
brittle.  Fig.  44  illustrates  these  requirements.  The  enamel 
plate  ABCD  rests  upon  sound  dentine.    The  rods  which  form 

FiQ.  43 


The  structural  requirements  for  a  strong  enamel  wall. 


the  cavosurface  angle  at  B  run  uninterruptedly  to  the  den- 
tine, and  their  inner  ends  rest  on  it  at  E.  The  rods  B,  E  are 
also  supported  by  a  portion  of  enamel,  ABE,  made  up  of 
rods  whose  inner  ends  rest  upon  the  dentine  and  whose 
outer  ends  are  covered  in  by  the  filling  material,  altogether 
supporting  the  marginal  rods  like  a  buttress.  And  the  cavo- 
surface angle  is  bevelled,  including  from  J  to  y  of  the  enamel 
6 


82     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

wall,  so  as  to  remove  the  sharp  corner  which  would  be  in 
danger  of  crumbling  in  under  an  instrument.  A  force  that 
causes  it  to  give  way  will  crack  it  through  its  entire  thickness. 
No  filling  material  or  substitute  for  the  lost  dentine  can 
restore  the  original  condition.     An  enamel  wall  should  be 


Fig.  44 


The  structural  requirements  for  a  strong  enamel  wall:  AB,  the  bevel  of  the 
cavosurface  angle.  The  rods  forming  the  margin  of  the  cavity  at  B  reach  the 
dentine  at  E,  and  are  supported  by  the  portion  ABE. 


considered  no  stronger  after  the  filling  is  inserted  than  it 
was  before.  Moreover,  when  the  dentine  has  been  decalcified 
or  destroyed  by  the  action  of  caries,  the  acid  which  has 
decalcified  the  dentine  has  also  acted  upon  the  enamel,  dis- 
solving the  cementing  substance  from  between  the  rods,  from 


EX  AM  EL  MUST  REST   UPON  SOUND  DENTINE     83 

within  outward,  often  to  a  great  extent,  and  the  structure 
is  very  imperfect.  Enamel  that  has  been  so  weakened  will 
not  withstand  the  force  of  mastication,  and  sooner  or  later 
will  crack  or  break  away  from  the  filling  material.  It 
should  be  removed  and  the  wall  formed  in  tissue  whose 
structure   is   perfect.      Occasionally   cases   arise   where   an 

Fig.  45 


Improperly  prepared   enamel  wall.      The  portion   ABC  has   the  inner   ends   of   the 
rods  cut  off   and  they  do  not  reach  the  dentine. 


operator  decides  to  leave  some  unsupported  enamel,  but  its 
weakness  and  the  possibility  of  restoring  it  if  it  breaks  away 
without  destroying  the  original  operation  must  always  be 
considered.  It  is  sometimes  supposed  that  it  is  only  neces- 
sary to  have  sound  enamel  resting  on  sound  dentine,  but  by 
looking  at  Figs.  45  and  46  it  will  be  seen  that  the  first  require- 


84     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

ment  may  be  present,  but  not  the  second.  In  these  illustra- 
tions the  enamel  plate  is  resting  on  sound  dentine,  but  the 
tissue  has  been  cut  in  such  a  way  that  the  inner  ends  of  the 
rods  have  been  cut  off.  The  rods  that  form  the  cavosurface 
angle  do  not  extend  to  the  dentine,  but  run  out  on  the 
cavity  wall  at  D,  and  the  portion  ABC  is  held  together  only 
by  the  cementing  substance.     This  is  not  strong  enough  to 


B 


\ 


Fig.  46 

c 


^ 


Improperly  prepared  enamel  -wall.      The  portion  ABC  is  not  supported   by  dentine. 


sustain  the  force  necessary  to  condense  the  filling  material 
or  the  forces  received  upon  the  surface  of  the  tooth  after 
the  filling  is  completed.  It  will  crack  on  the  line  of  the 
cementing  substance  and  chip  out.  The  inclination  of  the 
entire  wall  must  be  increased  to  a  little  more  than  to  reach 
the  rod  direction.  Such  a  wall  as  this  may  easily  be  made, 
in  preparing  a  cavity  wall,  with  a  stone  or  a  burr,  but  would 


THE  RODS  FORMING  THE  CAVOSURFACE  ANGLE     85 

be  unlikely  ever  to  be  formed  with  hand  instruments.  Such 
walls  as  this  account  for  the  chipping  of  many  margins  and 
the  failure  of  fillings  along  the  gingival  wall.  The  tissue 
is  cracked  to  pieces  in  inserting  the  filling  material,  and  the 
pieces  fall  out  later.  This  occurs  often  in  the  gingival  walls 
of  compound  cavities. 

Fig.  47 


Enamel  wall  cut  in  the  direction  of   the  rods, 
supported.      It  should  be  trimmed  i 


The  marginal  rods  are  not 
the  line  indicated. 


The  Rods  Forming  the  Cavosurface  Angle  Must  be  Sup- 
ported.— This  is  the  key  to  strong  enamel  walls.  The  more 
perfect  the  support  the  stronger  the  wall.  If  an  enamel  wall 
is  cut  exactly  in  the  direction  of  the  rods,  as  in  Fig.  47,  the 
rods  forming  the  margin  are  held  together  only  by  cementing 
substance,  and  a  comparatively  slight  force  on  the  surface 
in  the  direction  toward  the  cavitv  will  break  them  off.    If  the 


86     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

same  wall  is  trimmed,  as  indicated  by  the  line,  the  same  force 
would  do  no  dama^^e,  as  the  rods  which  receive  it  are  supported 
by  the  portion   which  is  covered   by  the  filling   material. 

Fig.  48 


The  tip  of   a  worn  incisor.      The  rods  forming  the  angle  at  A  reach  the  dentine  at  C, 
and  are  supported  by  the  piece  ABC. 


It  is  interesting  to  note  that  in  the  wearing  down  of  the 
enamel  by  use,  nature  provides  the  same  support  for  the 
rods  which  form  the  angle  of  the  worn  and  tooth  surfaces. 
Fig.  48  shows  the  tip  of  a  worn  incisor.    The  rods  at  A  reach 


CLASSES  OF  CAVITIES 


87 


the  dentine  at  B  and  are  supported  by  the  portion  ABC. 
When  caries  occurs  on  an  abraided  surface  it  starts  by  the 
rods  at  the  dento-enamel  junction,  chipping  out  and  forming 
a, protected  niche  for  the  lodgement  of  a  colony. 

Bevel  the  Cavosurface  Angle. — It  is  not  always  necessarj' 
to  bevel  the  cavosurface  angle  where  the  rods  are  inclined 
toward  the  cavity.  In  such  places  the  rods  forming  the 
margin  are  well  supported  and  the  angle  need  not  be  bevelled 
unless  it  is  so  sharp  that  it  would  be  in  danger  of  being 
injured. 


Fig.  49 


The  two  classes  of   cavities.      Those  with  the  rods  inclined  toward  the  cavity,  ana 
those  with  the  rods  inclined  away  from  the  cavity. 

There  are  two  reasons  for  bevelling  the  cavosurface  angle: 
(1)  To  protect  a  sharp  angle  from  injury;  (2)  to  gain  support 
for  the  marginal  rods.  The  first  occurs  where  the  enamel 
rods  are  inclined  toward  the  cavity,  the  second  where  they 
are  inclined  away  from  the  cavity. 

Classes  of  Cavities.— From  a  consideration  of  the  direction 
of  the  enamel  rods  in  the  tooth  crown,  and  the  positions  where 
caries  begins  on  the  enamel,  enamel  walls  may  be  divided, 
according  to  their  structural  type,  into  two  classes  (Fig.  49) : 


88    STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

1.  Those  in  which  the  enamel  rods  are  inchned  toward 
the  cavity,  characteristic  of  cavities  on  occlusal  surfaces  and 
cavities  beginning  in  fissures  and  pits. 

2.  Those  in  which  the  enamel  rods  are  inclined  away  from 
the  cavity,  characteristic  of  cavities  on  smooth  surfaces. 

In  the  firsi  class  it  is  comparatively  easy  to  obtain  a 
strong  margin,  and  this  is  fortunate,  for  when  the  filling  is 
completed  the  margin  will  be  subjected  to  the  full  force  of 
mastication.  In  the  second  it  is  comparatively  difficult  to 
obtain  a  strong  margin,  but  only  sufficient  strength  is 
required  to  withstand  the  force  of  condensing  the  filling 
material,  a6  after  the  filling  is  completed  it  will  be  obliged  to 
withstand  little  force  from  mastication. 

From  a  careful  observation  of  the  failures  of  filhngs  (his 
own  and  those  of  other  operators),  the  author  believes  a 
very  large  number  are  due  to  structurally  imperfect  enamel 
walls.  A  study  of  enamel  structure  as  related  to  cavity 
preparation  will  do  more  to  improve  the  quality  of  the 
operation  and  to  increase  the  facility  of  its  execution  than 
any  one  factor.  This  study  is  a  clinical  study  guided  by 
examination  of  the  microscopic  structure  of  the  tissue.  In 
operating  at  the  chair  the  detail  of  enamel  rod  direction  as 
it  is  applied  to  cavity  preparation  is  learned,  but  to  do  so 
hand  instruments  must  be  used  and  a  sufficient  knowledge 
of  the  tissue  must  have  been  acquired  to  think  of  it  always 
in  their  use  in  terms  of  its  structural  elements. 


CHAPTER  IX 
THE  PREPARATION  OF  TYPICAL  ENAMEL  WALLS 

The  steps  in  the  preparation  of  an  enamel  wall  are: 

1.  The  cleavage  of  the  enamel  until  the  outline  form  of 
the  cavity  is  reached. 

2.  The  trimming  of  the  enamel  walls. 

3.  The  preparation  of  the  margins. 

Every  enamel  wall  should  be  prepared  according  to  these 
steps.  The  first  not  only  removes  the  tissue  more  or  less 
disintegrated  and  weakened  by  caries,  but  also  places  the 
margin  of  the  filling  in  a  position  where  it  is  not  likely  to 
be  covered  by  the  growth  of  a  colony  of  bacteria.  It  also 
determines  the  direction  of  the  enamel  rods  so  that  the 
walls  can  be  completed  in  terms  of  its  structural  elements. 

The  second  step  is  accomplished  by  the  shaving  or  planing 
process,  and  should  always  increase  the  inclination  of  the 
entire  enamel  wall  slightly,  so  as  to  extend  a  little  beyond 
the  rod  directions,  and  remove  the  portions  that  have  been 
cracked  or  splintered  by  the  cleavage.  After  cleavage  the 
enamel  wall  will  usually  have  a  more  or  less  whitish  look. 
This  is  caused  by  the  cracking  of  the  cementing  substance 
between  the  rods.  The  light  is  refracted  by  the  air  in  these 
microscopic  spaces  and  imparts  this  whitish  or  snowy  look 
to  the  tissue.  These  portions  are  removed  by  planing  or 
shaving,  and  the  tissue  obtains  its  bluish  translucent 
appearance. 

The  third  step  is  also  accomplished  by  the  planing  process, 
and  should  be  carried  out  with  two  objects  in  mind:  (1)  To 
so  form  the  cavosurface  angle  that  the  tissue  will  not  be 
liable  to  injury  in  the  condensation  of  the  filling  material 
against  it,  and  (2)  to  leave  rods  whose  outer  ends  will  be 


90      PREPARATION  OF  TYPICAL  ENAMEL  WALLS 

covered  by  the  filling  material  to  support  those  which  form 
the  actual  margin  of  the  cavity. 

The  steps  in  the  })re])aration  of  enamel  walls  may  be  made 
more  clear  by  photomicrographs.  Plate  III  shows  a  portion 
of  enamel  close  to  a  carious  cavity  which  is  to  be  extended 
to  the  left.  The  chisel  is  placed  close  to  the  margin  and  the 
portion  is  split  off.  The  wall  then  appears  whitish,  for,  as  is 
seen,  the  cementing  substance  has  cracked  in  several  places, 
disturbing  the  structure,  and  in  several  places  rods  have 
been  broken  across.  The  wall  must  now  be  planed  so  as  to 
increase  the  inclination  of  the  entire  wall  slightly,  and 
finally  the  cavosurface  angle  must  be  bevelled,  involving 
from  ^  to  ^  of  the  thickness  of  the  enamel  wall  to  give 
support  to  the  rods  forming  the  margins.  In  this  case  the 
rods  are  straight  and  parallel,  but  in  Plate  IV  they  are  twisted. 
If  the  dentine  is  removed  from  under  this  enamel  and  the 
chisel  placed  as  indicated,  the  portion  will  be  split  out,  but 
not  only  has  the  tissue  been  splintered,  but  a  considerable 
portion  is  left  in  which  the  rods  have  been  broken  across. 
By  feeling  of  the  margin  with  the  chisel  this  can  easily  be 
determined,  and  the  angle  of  the  wall  must  be  increased  by 
planing  so  as  to  leave  the  wall  in  the  position  shown  in  Plate 
IV,  3,  and  finally  the  cavosurface  angle  must  be  bevelled. 

Preparation  of  Simple  Occlusal  Cavities. — Caries  often 
begins  in  the  mesial  and  distal  pits  of  the  upper  bicus- 
pids, and  in  preparing  the  cavities  for  filling  they  must 
be  united.  Fig.  50  is  a  buccolingual  section  through  a 
first  superior  bicuspid.  Suppose  caries  has  reached  the 
dento-enamel  junction  in  both  the  mesial  and  distal  pits, 
and  the}'  are  to  be  united  along  the  groove.  A  small 
spear  drill  is  carried  into  the  mesial  pit  until  the  dento- 
enamel  junction  is  reached,  then  a  small  inverted  cone  burr 
is  carried  into  the  dentine  just  under  the  enamel  and  drawn 
from  the  dentine  to  the  surface  of  the  enamel.  When  a 
narrow  cut  has  been  made  from  the  mesial  to  the  distal 
pit,  a  chisel  placed  at  the  edge  of  the  opening  wdll  split  out 
the  enamel  as  indicated  in  Fig.  51.  Xow  the  walls  must  be 
planed  so  as  to  bring  the  buccal  and  lingual  walls  into  the 


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Preparation  of  Enamel  Wall  in  Gnarled   Enamel. 

1,  enaniel  wall  as  cleaved,  showing  breaking  across  rods  and  slivering 
at  a.  2,  wall  as  sniooth,  but  not  extended  to  remove  short  rods  whose 
inner  ends  are  cut  off  at  b.  3,  wall  extended  and  trinimed  to  a  position 
of  strength.  D,  dentine;  De,  dento-enamel  junction;  c,  cavosurfaee  angle; 
6,  point  where  inner  ends  of  rods  are  cut  off;  a,  slivering  of  the  tissue. 
(About  80  X) 


PREPARATIOX  OF  SIMPLE  OCCLUSAL  CAVITIES     91 

axial  plane,  and  the  structural  requirements  will  have  been 
completed  (P'ig.  52).  Fig.  53  shows  the  relation  of  the 
cavity  to  the  crown. 

Fig.  50 


Occlusal  Assure  in  an   upper   bicuspid,  showing   direction  of   rods.        (About  80  X) 


92      PREPARATION  OF  TYPICAL  ENAMEL  WALLS 

It  has  often  been  advised  to  allow  the  filling  to  extend  on 
to  the  natural  slopes  of  the  cusps,  as  indicated  in  Fig.  54. 

Fig.  51 


/ 


# 


The  same  section   as   Fig.  51.  sbowing  the  position   of  the  chisel   in   cleaving   the 
enamel  to  open  the  cavity. 


PREPARATION  OF  SIMPLE  OCCLUSAL  CAVITIES     93 


Fig.  52 


Preparation  of  enamel  walls  in  occlusal  fissure  cavities  (the  same  as  Figs.  50  and  51). 


94         PREPARATION  OF   TYPICAL  ENAMEL   WALLS 

Fig.  53 


The  relation  of   the  cavity  to  the  crown  (the  same  as  Figs.  50  and  51). 


The  trimming  of  the  walls  instead  of  lapping  the  filling  material  on  the  slope  of  the  cusps. 


PREPARATIOX  OF  SIMPLE  OCCLUSAL  CAVITIES     95 

It  will  be  seen,  however,  that  a  stronger  enamel  wall  and  a 
stronger  edge   of  filling   material   will   be   obtained   if  the 


Fig.  55 


Caries  beginning  in  an  occlusal  defect  of   a  molar.      (About  SO  X) 

enamel  wall  is  bevelled  to  the  point  where  the  margin  of  the 
filling  is  desired  and  the  filling  finished  to  this  position. 


96      PREPARATION  OF  TYPICAL  ENAMEL  WALLS 


Fig.  55  shows  a  buccolingual  section  through  a  molar 
with  a  small  cavity  in  a  mesial  pit.    Caries  has  undermined 


Fig.  56 


The  preparation  of  the  enamel  walls  of  the  cavity  shown  in  Fig.  55 

the  enamel  slightly  toward  the  buccal,  but  has  attacked  the 
enamel  on  the  surface,  extending  toward  the  lingual  farther 


PREPARATION  OF  SIMPLE  OCCLUSAL  CAVITIES     97 


than  the  enamel  has  been  undermined  at  the  dento-enamel 
junction.  Applying  the  chisel  to  the  surface,  the  undermined 
enamel  is  split  away,  as  is  indicated  in  Fig.  56.  The  buccal 
wall  is  planed  until  it  is  in  the  axial  plane,  and  the  cavo- 
surface  angle  bevelled.  It  is  not  necessary  to  extend  the 
cavity  to  the  lingual  beyond  the  point  where  sound  dentine 
is  reached,  but  the  disintegrated  enamel  on  the  surface  must 
be  removed.  The  enamel  wall  is,  therefore,  inclined  about 
6  centigrades  lingually  from  the  axial  plane,  and  it  is  not 


Fig.  57 


^^\ 
A 


The  relation  nf  tl,e  cavity  to  the  crown  (the  same  section  as  shown  in  Figs.  55  and  56). 

necessary  to  bevel  the  cavosurface  angle.  The  rods  are 
inclined  toward  the  cavity  and  the  rods  forming  the  margins 
are  well  supported,  and  the  cavosurface  angle  is  not  so 
sharp  as  to  be  endangered  in  condensing  filling  material. 
Fig.  57  shows  the  relation  of  the  cavity  to  the  crown. 

All  occlusal  defects  should  be  filled  as  soon  as  the  decay 
has  reached  the  dento-enamel  junction,  as  all  progress  of 
the  disease  beyond  that  point  requires  sacrifice  of  tissue 
which  otherwise  would  be  saved,  and  the  enamel  wall  becomes 
less  and  less  strong.  Fig.  58  shows  a  much  more  exten- 
7 


98       PREPARATION  OF   TYPICAL  ENAMEL  WALLS 

Fig.  58 


A  larger   cavity  in  (he  occlusal   surface  of  a  molar.       The  position  of   the  chisel  in 
opening  the  cavity 

FiQ.  59 


The  preparation  of  the  cavity  shown  in  Fig    .58. 


PREPARATION  OF  SIMPLE  OCCLUSAL  CAVITIES     99 


Fig.  60 


A  gingival   third   caviti'   in  a   bicuspid,  showing  the    cleavage   of   the   occlusal    and  i 

gingival  walls  as  cleaved.  _ 


100     PREPARATION  OF  TYPICAL  ENAMEL  WALLS 

Fig.  61 


The  preparatiun  of  the  cavity  shown  in  Fig.  60. 


GINGIVAL  THIRD  CAVITIES 


101 


sive  occlusal  cavity,  one  that  has  been  neglected  until 
the  enamel  has  been  broken  in,  and  as  a  result  there  was 
much  unnecessary  loss  of  tooth  structure.  The  chisel  is 
applied  to  the  surface  as  indicated,  and  the  undermined 
enamel  broken  down  until  the  sound  dentine  is  reached.  On 
the  buccal,  the  enamel  wall  is  cut  to  the  axial  plane,  and  the 
cavosurface  angle  bevelled.  If  the  decay  in  the  dentine  had 
reached  the  tip  of  the  dentine  cusp,  it  would  be  necessary 
to  remove  the  tip  of  the  enamel  cusp  and  incline  the  wall 


A  gingival  third  cavity  in  a  molar. 


about  8  centigrades  buccally  from  the  axial  plane,  in  order 
to  obtain  a  strong  wall,  and  then  the  cusp  would  be 
replaced  by  filling  material.  On  the  lingual  the  undermined 
enamel  is  removed,  and  the  wall  inclined  slightly  lingually 
from  the  axial  plane  and  the  cavosurface  angle  bevelled  a 
little.  Fig.  59  shows  the  relation  of  the  cavity  to  the  crown. 
Gingival  Third  Cavities. — Fig.  60  is  a  buccolingual  section 
of  a  superior  bicuspid,  showing  a  break  in  the  enamel  in 


Fig.  G3 


'1.    Wall  as_cleaved. 
Fig.  64 


2.    Wall  as  trimmed. 
Preparation  of  occlusal  wall  of  Fig.  62.      (About  70  X) 


GINGIVAL  THIRD  CAVITIES 


103 


the  position  of  a  gingival  third  cavity.    The  occlusal  wall  is 
cleaved  to  find  the  enamel  rod  direction,  then  planed  to 


Fig.  65 


A  ca-Wty  in  the  lingual  pit  of  a  lateral  incisor.      The  position  of  the  chisel  in 
opening  the  cavity. 


104     PREPARATION  OF  TYPICAL  ENAMEL  WALLS 

increase  the  inclination  slightly,  leaving  it  about  8  centigrades 
occlusally  from  the  horizontal  plane,  and  the  cavosurface 
angle  bevelled  to  obtain  support  for  the  marginal  rods. 
The  gingival  wall  is  prepared  in  the  same  way,  inclined 
gingivally  about  G  centigrades  from  the  horizontal  plane, 
and  the  cavosurface  angle  bevelled.  Fig.  Gl  shows  the  walls 
prepared. 

Fig.  66 


The  preparation  of   the  gingival  wall  of   the  cavity  shown  in  Fig.  65. 

Fig.  62  is  a  similar  section  from  a  molar.  After  chopping 
away  the  occlusal  wall  until  the  cavity  has  been  extended 
to  the  point  of  greatest  convexity  of  the  surface,  the  wall  is 
seen  to  be  in  the  condition  shown  in  Fig.  G3.  Near  the  sur- 
face some  rods  have  broken  across,  and  near  the  dento-enamel 
junction  the  same  thing  has  happened,  but  in  the  rest  of  the 
distance  the  cleavage  has  followed  the  enamel  rod  direction. 
The  inchnation  of  the  wall  is  increased  by  planing  until 
this  roughness  has  been  removed,  and  then  the  cavosurface 
angle  is  bevelled  to  support  the  marginal  rods,  and  prepara- 
tion is  complete,  as  shown  in  Fig.  G4. 


GINGIVAL  THIRD  CAVITIES 


105 


Fig,  65  shows  a  cavity  in  the  Ungual  pit  of  a  superior 
lateral  incisor.     Caries  has  undermined  the  enamel  to  a 


Fig.  67 


The  preparation  of   the  cavity  ishown  in  Fig.  65. 


lOG     PREPARATION  OF   TYPICAL  ENAMEL   WALLS  \ 

considerable  extent,  and  the  cavity  will  have  to  be  larger  ! 

than  would  otherwise  have  been  necessary.     Placing  the  • 

chisel  close  to  the  occlusal  margin,  as  indicated,  the  enamel  i 

is  chij^ped  away  in  that  direction  and  around  the  circum-  | 
ference.     On  the  lingual  wall  the  chisel  may  be  reversed 

and  used  with  a  pulling  motion,  like  a  hoe.     In  this  way  i 

the  undermined   enamel   is  chipped   away  and   the   tip  of  1 

the  marginal  ridge  removed.     The  wall  is  then  planed  into  1 

the  horizontal  plane  and  the  cavosurface  angle  })evelled.  \ 
Fig.  (36  shows  the  structure  of  the  gingival  wall,  and  Fig.  67 
the  relation  to  the  crown. 


CHAPTER  X 
STRUCTURAL   DEFECTS   IN   THE   ENAMEL 

The  formation  of  enamel  begins  at  the  dento-enamel 
junction,  and  the  tissue  is  laid  down  from  within  outward, 
so  that  the  enamel  in  contact  with  the  dentine  is  formed 
first  and  the  surface  of  the  crown  last.  Enamel  formation 
begins  at  several  points,  for  each  crown,  the  exact  number 
and  position  of  which  has  been  the  subject  of  much  investi- 
gation. When  enamel  formation  begins,  these  points  are 
close  together,  but  they  are  carried  farther  apart  by  the 
growth  of  the  dental  papilla,  and  are  not  united  for  some 
time.  The  separate  enamel  caplets  unite  first  at  the  dento- 
enamel  junction,  and  as  the  formation  of  the  thickness  of 
the  enamel  progresses  at  these  lines  of  union,  there  is  always 
more  or  less  disturbance  in  structure.  Even  where  the  union 
seems  perfect,  sections  will  show  more  or  less  disturbance  of 
enamel  rod  direction,  arrangement  of  the  rods,  and  relation 
to  the  cementing  substance. 

Every  operator  and  student  of  dental  anatomy  is  familiar 
with  the  developmental  lines.  On  the  occlusal  surfaces  they 
are  usually  marked  by  well-defined  grooves,  but  upon  the 
axial  surfaces  the  grooves  may  be  very  slight,  scarcely  more 
than  slight  depressions  of  the  surface,  and  consequently 
they  are  not  thought  of.  It  will  be  found,  however,  that  on 
these  lines  there  is  less  perfect  enamel  structure,  and  con- 
sequently the  tissue  is  not  as  strong,  and  these  lines  must  be 
avoided  in  the  preparation  of  enamel  walls.  The  cause  of 
disturbance  of  structure  will  be  better  understood  after  study 
of  the  development  of  the  tooth  germ  and  the  formation 
of  enamel  in  the  chapter  on  Dental  Embryology,  but  some 


108         STRUCTURAL  DEFECTS  IN  THE  ENAMEL 

details  of  the  cause  should  be  touched  upon  here.  The  study 
of  the  diagrams  of  the  growth  of  the  tooth  crown  will  illustrate 
the  conditions  (see  Chapter  XXVII),  and  shows  a  bucco- 
lingual  section  through  the  tooth  germ  of  a  bicuspid  just 
before  the  formation  of  the  dentine  and  the  enamel  begins. 
The  odontoblasts  (dentine  forming  cells)  and  the  ameloblasts 
(enamel  forming  cells)  are  in  contact  at  what  will  be  the 
dento-enamel  junction.  The  odontoblasts  form  dentine 
on  their  outer  surface,  beginning  at  the  tip  of  the  dentine 
cusp,  and  progress  from  without  inward  and  extend  down 
the  slopes  of  the  cusps.  The  ameloblasts  form  enamel  on 
their  inner  surface  and  progress  from  within  outward  and 
down  the  slopes  of  the  cusps.  In  this  way  little  caplets  of 
dentine  covered  by  enamel  are  formed  over  the  horns  of  the 
dental  papilla;  the  caps  are,  of  course,  thickest  where  forma- 
tion has  been  going  on  longest.  While  these  caps  are  forming, 
the  dental  papilla  is  increasing  in  size,  and  so  they  are  carried 
farther  and  farther  apart  (Figs.  68  to  73).  As  soon  as  the 
calcifications  reach  each  other  at  the  dento-enamel  junction 
and  unite,  the  increase  in  the  diameter  of  the  dental  papilla 
ceases.  The  layer  of  ameloblasts,  which  are  tall  col- 
umnar cells,  now  cover  the  surface  of  the  enamel  and 
receive  their  nourishment  and  the  materials  for  the  forma- 
tion of  enamel  from  the  blood  supply  through  the  stratum 
intermedium.  As  the  blood  supply  comes  from  above,  it  is 
evident  that  the  cells  high  up  along  the  slopes  of  the  cusps 
will  receive  most,  while  those  at  the  bottom  of  the  groove 
get  what  is  left.  The  formation  is,  therefore,  more  rapid 
along  the  slopes  and  less  rapid  at  the  point  of  union.  As 
growth  continues,  this  difference  in  supply  increases,  and 
accordingly  formation  at  the  bottom  of  the  groove  is  first 
slowed  and  finally  stopped,  and  the  result  is  a  defect.  The 
taller  the  cusps  the  greater  will  be  the  interference  and  the 
deeper  the  defective  groove.  In  studying  sections  (Figs.  74 
to  78)  it  is  very  noticeable  that  teeth  with  long  pointed 
cusps  have  more  open  grooves,  and  the  defect  often  extends 
almost  to  the  dento-enamel  junction. 


STRUCTURAL  DEFECTS  IN   THE  ENAMEL        109 
Fig.  68 


*5''' « 


Fig,  69 


w 


Diagram  showing  the  growth  of   the  crown  of   a  bicuspid. 


110         STRUCTURAL   DEFECTS  IN   THE  ENAMEL 


Fig.  70 


Fig.  71 


Diagram  showing  the  growth  of  the  crown  of  a  bicuspid. 


STRUCTURAL  DEFECTS  IN   THE  ENAMEL        IJl 
Fig.  72 


Fig.  73 


Diagram  showing  the  growth  of  the  crown  of  a  bicuspid. 


112         STRUCTURAL  DEFECTS  IN   THE  ENAMEL 

Fig.  74 


The  sectioii  from  which  Figs.  68  to  73  were  drawn:  A,  tip  of  dentine  cusp,  B, 
lines  showing  little  caps  of  enamel  formed  before  calcifications  from  separate  centres 
united;   C,  lines  showing  amount  of  enamel  formed  when  calcifications  united. 

Fig.  75 


Occlusal  defect  from  an  old  tooth. 


Fig.  76 


A  deep  open  groove. 
Fig.  77 


A  shallow  groove. 


114       STRUCTURAL  DEFECTS  IN   THE  ENAMEL 

The  bands  of  Retzius,  which  are  the  incremental  Hnes  of 
the  enamel,  should  be  studied  about  these  grooves.  It  will 
be  seen  that  they  always  dip  down  around  the  groove,  and 
that  more  enamel  has  been  formed  between  one  band  (Figs. 
84  and  85)  and  the  next  on  the  slope  of  the  cusps  than  at 
the  bottom  of  the  groove.  In  teeth  with  very  flat,  low  cusps 
the  closure  of  the  grooves  may  be  very  perfect,  leaving  only 
a  slight  depression  (Fig.  77). 

Fig.  78 


A  very  deep  groove,  showing  the  effect  of   caries  at  the  bottom. 


The  importance  of  these  defects  as  positions  of  beginning 
caries  cannot  be  overestimated,  as  they  furnish  ideal  con- 
ditions in  areas  that  would  otherwise  be  immune,  and  they 
are  the  positions  in  which  the  attacks  of  caries  are  first 
manifested.    These  occlusal  grooves  appear  in  great  variety. 


STRUCTURAL  DEFECTS  IN   THE  ENAMEL         115 


Fig.  79 


The  pit  in  a  lateral  incisor  filled  with  coronal  cementum.      Interglobular  spaces  are 


Fig.  80.— Occlusal  surface  of  the  lower  third  molar,  showing  the  grooves. 
Fig.  81.— The  same  tooth  sliced  for  sectioning:     1,  the  piece  from  which  the  sec- 
tion shown  in  Figs.  82  and  83  was  ground. 


116 


STRUCTURAL  DEFECTS  IN   THE  ENAMEL 


Some  are  simply  shallow  open  grooves,  in  which  the  surface 
of  the  enamel  is  perfect  (Fig.  74);  some  are  very  deep  and 
entirely  empty  (Figs.  75,  7G,  and  78) ;  others  are  apparently 
filled  with  a  granular,  more  or  less  structureless  calcified 
material  which  appears  to  have  been  deposited  in  the  groove 
after  the  enamel  was  completed  (Figs.  79,  84,  and  85).  This 
is  probably  of  the  nature  of  cementum.  It  was  formed  after 
the  enamel  was  completed,  but  while  the  tooth  was  enclosed 


Fig.  82 


The  section  ground  from  1,  Fig.  81,  showing  the  depth  of  the  fissure. 


in  its  follicle  in  the  crypt  in  the  bone.  It  is  to  be  compared 
with  the  coronal  cementum  that  is  characteristic  of  the 
complex  grinding  teeth  of  the  ungulates  and  other  herbivor- 
ous animals.  A  study  of  these  defects  furnishes  the  basis 
for  the  operative  rule  that  "all  grooves  must  be  cut  out  to 
the  point  where  the  margin  will  be  on  a  smooth  surface." 
For  if  they  are  not,  a  defect  will  be  left  at  the  margin  of  the 
cavity  which  offers  ideal  conditions  for  the  beginning  of  a 
new  decay.     When  caries  begins  in  such  a  defect  at  the 


STRUCTURAL  DEFECTS  IN   THE  ENAMEL 


117 


margin  of  a  filling,  it  progresses  at  the  bottom  of  the  defect 
until  the  dento-enamel  junction  is  reached,  and  then  extends 
in  the  dentine  and  may  destroy  the  entire  cro\yn  without 


Fig.  S3 


Higher  magnification  of  the  fissure  shown  in  Fig.  82.     (About  60  X) 


lis       STRUCTURAL  DEFECTS  IN   THE  ENAMEL 

showing  upon  the  surface  (see  Chapter  XII).  The  extent 
of  these  defects  is  much  greater  than  would  be  supposed 
from  the  observation  of  the  teeth  in  the  mouth.  Fig.  80 
shows  the  occlusal  view  of  a  lower  third  molar,  extracted 
because  of  disease  of  the  peridental  membrane,  from  a  man 
aged  about  forty  years.  Examining  these  grooves  with  a 
fine-pointed  explorer,  it  would  not  stick  any  place.  No 
operator  would  think  of  cutting  them  out  and  filling  them. 


Fig.  84 


An  occlusal  defect  in  a  worn  tooth.    The  fissure  is  filled  with  coronal  cementum. 


The  crown  w^as  sawed  through  from  buccal  to  lingual,  as 
shown  in  Fig.  81,  and  the  piece  marked  1  is  shown  in 
Figs.  82  and  83.  The  grooves  are  open  two-thirds  of  the 
distance  to  the  dento-enamel  junction,  and  show  slight 
action  of  caries.  Suppose  caries  had  started  in  the  central 
pit,  and  a  small  round  filUng  had  been  made,  open  defects 
would   be   left   at   the   margin   where   every  groove  radi- 


STRUCTURAL   DEFECTS  IX   THE  ENAMEL         119 

ated  from  the  central  cavity,  and  these  would  be  just  as 
liable  to  recurrent  decay  as  it  was  originally,  and,  if  caries 
occurred,  it  would  progress  at  the  depth  of  the  groove, 
reach  the  dento-enamel  junction,  and  progress  in  the  dentine, 


Fig.  85 


Higher  magnification  of  Fig.  84.     The  fissure  filled  with  granular  calcified  material. 
Notice  the  direction  of  the  bands  of  Retzius  around  the  fissure. 


until  the  occlusal  enamel  was  so  undermined  that  it  would 
break  in  under  the  force  of  mastication.  On  the  other  hand, 
if  the  grooves  are  cut  out  to  a  point  where  the  cavity  margin 


120        STRUCTURAL  DEFECTS  IN  THE  ENAMEL 

will  be  on  a  smooth  surface,  there  is  no  possibility  of  recur- 
rent caries  if  the  filling  material  is  properly  inserted.  This 
one  illustration,  which  might  be  duplicated  a  thousand  times, 


Structural  defects  in  developmental  grooves  on  axial  surfaces. 


] 

Fig.  87 

B 

.-;: 

A 

A-C 

"i; 

V"5-^ 

.!_ 

-A 

Structural  defects  in  developmental  grooves  on  axial  surfaces. 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL 


121 


therefore  is  the  rational  basis    for  the  rule,  "All  grooves 
must  be  cut  out  to  their  end." 

Caries  does  not  occur  in  all  open  grooves.  Fig.  76  shows 
an  open  groove  in  a  section  from  a  tooth  in  which  the  wear 
indicates  that  it  was  not  from  a  young  person,  but  most  of 
the  grooves  that  escape  are  not  open,  but  more  or  less  entirely 
filled  with  structureless  calcified  matter  or  coronal  cementum. 
Figs.  SO,  S4,  and  So  are  very  good  illustrations  of  this  class 
of  grooves. 


Fig.  88 


Defects  on  the  axial  surface  in  the  enamel. 


The  condition  in  pits  from  which  grooves  extend,  as  the 
lingual  pits  of  incisors  and  the  buccal  pits  of  molars,  show 
the  same  condition  as  the  grooves,  except  that  the  defect 
is  both  broader  and  deeper.  But  pits  that  are  sometimes 
found  on  the  tips  of  cusps  and  on  smooth  surfaces  show  an 
entirely  different  structural  condition,  and  will  be  considered 
under  atrophy  in  Chapter  XII. 


122  STRUCTURAL  DEFECTS  IN   THE  ENAMEL 

In  places  where  the  union  of  the  enamel  plates  seems  per- 
fect, as,  for  instance,  on  the  labial  surface  of  the  incisors  or 
the  buccal  surface  of  the  bicuspids,  and  the  line  of  union 
is  marked  only  by  a  slight  depression  of  the  surface,  the 
section   will  show  disturbance   of   structure.     Fig.   86,   a 


Fig.  89 


A  section  through  such  a  defect  as  that  shown  in  Fig.  88.    (About  80  X ) 


drawing  made  by  Dr.  Black  a  good  many  years  ago,  shows 
such  a  position.  At  the  surface  the  rods  and  their  arrange- 
ment seem  very  perfect,  but  from  a  point  about  one-third 
the  distance   to  the  dento-enamel  junction  there  are   no 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL        123 

rods  at  all,  but  apparently  a  number  of  calcospherites  in 
a  granular  calcific  substance.  In  Fig.  87,  another  of  Dr. 
Black's  illustrations,  the  rods  are  very  irregular,  and  are 
separated  by  large  areas  of  structureless  calcified  material. 
Grooves  are  often  found  in  unusual  or  atypical  positions. 
Fig.  88  shows  a  groove  running  over  the  mesial  marginal 
ridge  and  down  on  the  mesial  surface.  Fig.  89  shows  a 
section  through  such  a  defect.  Notice  the  folding  of  the 
enamel  into  the  dentine  and  the  disturbance  of  the  rods 
about  the  groove  and  between  its  base  and  the  dentine. 


CHAPTER  XI 

SPECIAL  AREAS  OF  WEAKNESS   FOR  ENAMEL   MARGINS 

There  are  certain  positions  which  in  the  perfect  crown 
are  areas  of  great  strength,  but  which,  because  of  the  pecu- 
liar structure  of  the  tissue  in  these  places,  become  areas 
of  weakness  when  cavity  margins  are  made  in  them.  The 
treatment  of  beginning  caries  would  lead  to  no  failures  in 
these  positions,  for  cavity  margins  would  never  be  extended 
into  them,  except  in  the  treatment  of  burrowing  caries  and 
neglected  cases.  The  extension  of  caries  at  the  dento-enamel 
junction  often  requires  the  extension  of  the  margin  into  the 
area  of  danger.  In  considering  these  areas  and  in  the  prepa- 
ration of  cavities,  as  well  as  the  areas  of  imperfect  structure 
considered  in  Chapter  X,  it  is  important  to  place  as  much 
emphasis  on  the  necessity  of  7iot  extendirig  cavity  margins 
into  the  areas  of  weakness,  as  on  cutting  away  the  dangerous 
area  and  leaving  the  margin  in  a  safe  position,  when  the 
area  cannot  be  avoided. 

In  considering  the  relation  of  the  enamel  and  dentine,  and 
in  studying  the  arrangement  of  the  enamel  rod  direction  in 
the  "architecture"  of  the  tooth  crown,  it  has  been  pointed 
out  that  the  dentine  cusps  and  the  dentinal  marginal  ridges 
are  not  directly  under  the  corresponding  points  on  the  sur- 
face of  the  enamel,  but  are  nearer  to  the  axis  of  the  tooth. 
The  areas  on  the  surface  of  the  enamel,  from  the  point 
directly  over  the  tip  of  the  dentine  cusp  or  ridge  to  the  tip 
of  the  enamel  cusps  or  ridges,  become  areas  of  weakness 
when  a  cavity  is  extended  into  them. 

Fig.  90  is  a  photomicrograph  of  a  buccolingual  section  of 
a  superior  bicuspid,  and  Fig.  91  is  a  higher  magnification 
of  the  same,  made  to  illustrate  the  condition.     It  will  be 


AREAS  OF  WEAKXESS  FOR  ENAMEL  MARGINS     125 

seen  that  if  decay  has  extended  at  the  dento-enamel  junction 
to  the  tip  of  the  dentine  cusp,  and  the  enamel  walls  were 
left  in  the  axial  plane,  the  rods  which  form  the  surface  of 


Fig.  90 


B  A 

Buccolingual  section  of  upper  bicuspid.      Enamel  is  broken  from  grinding.     A  to  B, 
area  of  weakness  for  enamel  margin.     (About  20  X) 


the  enamel  from  the  margin  of  the  cavity  to  the  tip  of  the 
cusp  "are  not  supported  by  dentine/'  and  would  be  likely 
to  be  broken  and  fall  aw^ay,  leaving  a  defect  at  the  margin 


Fig.  91 


Enamel  over  tip  of  dentine  cusp:  D,  dentine  cusp.     (About  80  X)     From  same 
section  as  Fig.  90. 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS     127 

of  the  filling.  If  decay  beginning  in  the  groove  or  pit  has 
extended  only  to  point  C,  Fig.  90,  the  wall  may  be  trimmed 
in  the  axial  plane  and  an  ideal  wall  produced;  but  if  it  has 
reached  point  D,  Fig.  90,  it  must  be  inclined  bucally,  so  as 
to  remove  the  tip  of  the  cusp,  as  indicated  in  the  dotted 
line,  and  the  cusp  restored  by  the  filling  material.  The 
region  of  the  surface  indicated  by  A-B,  while  an-  area  of 


Fig.  92 


A  bicuspid  cut  for  sectioning.     Sections  were  ground  from  the  positions  marked  by 
the  lines  1,  2,  3,  4  in  B,  and  are  sho-svn  in  Figs.  93,  94,  95,  and  96. 


strength  in  the  perfect  tissue,  becomes  a  position  of  weak- 
ness when  cavity  margins  are  extended  into  them.  A 
careful  observer  wiU  find  many  failures  that  are  the 
result  of  bad  enamel  wall  preparation  in  these  areas.  The 
same  conditions  exist  in  the  region  of  the  marginal 
ridges.  Figs.  96  and  97  show  the  mesial  marginal  ridge 
of  a  superior  bicuspid.  If  this  is  filled  before  the  destruc- 
tion of  dentine  has  extended  beyond  the  point  A,  the  mesial 
wall  ma^  be  cut  in  the  axial  plane  as  indicated;  but  if  it 
has  reached  the  tip  of  the  dentine  ridge  at  point  J5,  it  must  be 
inclined  mesial ly,  so  as  to  reach  the  tip  of  the  enamel  ridge. 


128      AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 


FiQ.  93 


Section  ground  from  1,  Fig.  92,  through  the  mesial  oblique  ridge.    (About  30  X) 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS     129 


Fig.  94 


Section  ground  from  2.  Fig,  92      (About  30  X), 


130      AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 


Fig.  95 


Section  ground  from  3,  Fig  92,  through  distal  marginal  ridge.     (About  20  X) 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS     131 

Fig.  96 


Section  ground  from  Fig.  92,  through  mesial  part  and  marginal  ridge.  If  caries 
has  extended  at  the  dento-enamel  junction  to  A,  the  -wall  may  be  in  the  axial  plane; 
if  it  has  reached  B,  the  wall  must  be  inclined  as  indicated  by  the  dotted  line. 
(About  30  X) 


132      AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 

Figs.  98,  99,  and  100  show  the  distal  marginal  ridge  in  a  sec- 
ond molar.  Notice  the  inclination  of  the  rods  from  the  tip  of 
the  dentine  ridge.     If  decay  has  reached  this  point  the  wall 


Fig.  97 


A  higher  magnification  of  Fig.  96,  showing  enamel  rod  directions  in  the  region 
of  the  marginal  ridge. 


must  be  inclined  distally,  so  as  to  reach  the  rod  direction, 
or  a  frail  margin  will  be  left  and  one  which  will  not  sustain 
the  force  of  mastication.     Neglected  caries  in  the  lingual 


AREAS  OF   WEAKXESS  FOR  EXAMEL  MARGIXS     133 

pits  of  incisors  often  present  the  same  conditions.  Fig.  65 
shows  a  section  through  such  a  pit  in  a  lateral  incisor,  and 
Fig.  06  shows  the  gingival  wall.  If  this  were  prepared  by 
inclining  the  gingival  wall  only  slightly,  a  very  frail  wall 


Fig.  9S 


An  upper  molar,  showing  the  position  of  the  section  shown  in  Figs.  99  and  100. 

Fig.  99 


The  section  ground  from  Pig.  9S. 


would  be  left.  It  should  be  cut  down  to  the  horizontal  plane, 
as  indicated,  and  the  marginal  ridge  restored  by  the  filling 
material r  The  same  conditions  are  often  encountered  in 
the  preparation  of  simple  cavities  in  the  mesial  or  distal 
surfaces   of  incisors,  when  caries  has  followed  the  dento- 


134      AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 


Fig.  100 


The  distal  marginal  ridge,  showing  the  enanael  rod  direction.    (About  30  X) 


AREAS  OF  WEAKXESS  FOR  ENAMEL  MARGINS     135 


Fig.  101 


A  B 

An  upper  bicuspid,  showing  the  position  of  the  section  shown  in  Fig,  102. 


Fig.  lO: 


Section  from  the  central  piece  shown  in  Fig.  101,  showing  the  dinection  of  cleavage 
on  the  mesial  surface  and  the  effect  of  caries  on  the  tissue. 


136      AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 

enamel  junction  toward  the  lingual.  Fig.  1 03  shows  a  superior 
central  incisor  from  which  sections  were  cut  as  indicated. 
Suppose  caries  to  have  begun  in  the  region  of  the  contact 
point  and  to  have  extended  to  the  point  a.  If  the  lingual 
enamel  wall  were  prepared  at  the  line  A,  Fig.  105,  a  verj^  frail 

Fig.  103 


A  superior  central  incisor,  showing  the  position  of  sections  in  Figs.  104,  105,  and  106. 

Fig.  104 


Section  1,  Fig.  103,  showing  the  enamel  worn  from  the  marginal  ridges. 


wall  would  result.  Force  coming  upon  this  wall  from  the 
Hngual,  by  the  occlusion  of  the  lower  incisors,  would  be  likely 
to  break  out  or  crack  a  triangular  piece  of  enamel,  and  the 
filling  would  fail  along  the  lingual  wall.  If,  however,  the  wall 
be  laid  in  the  line  at  B,  a  strong  wall  is  produced,  against 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS     137 

which  gold  can  be  properly  condensed  without  danger,  and 
which  will  withstand  the  force  of  occlusion. 

Dentists  are  often  tempted  to  prepare  simple  cavities  in 
the  mesial  surfaces  of  first  and  second  bicuspids  and  occa- 
sionally in  the  molars.  If  this  is  ever  done,  it  must  be  with 
the  full  knowledge  both  of  the  liability  of  recurrence  of  caries 
and  the  structure  of  the  enamel,  for  experience  shows  that 

Fig.  105 


Section  2,  Fig,  103,  showing  position  of  weak  and  strong  lingual  walls. 


such  operations  usually  fail,  either  by  recurrence  of  caries 
at  the  buccogingival  or  linguogingival  angles,  or  by  the 
breaking  out  of  the  enamel  of  the  marginal  ridge.  Fig.  107 
shows  the  mesial  surface  of  a  superior  bicuspid.  There  was 
a  white  spot  on  the  contact  point,  but  no  actual  cavity,  as 
the  enamel  rods  had  not  fallen  out.  A  section  was  ground 
through  this  point,  and  Fig.  108  shows  a  photomicrograph 


Fig.  100 


•  "^  '^'' i^^B 

m^^ 

d  \ 

^^^^^^HK'' 

^^^^^^^^^ 

//^^ 

^^^^^^^^^^^^^^^B^M^^^^^Ika 

A  higher  magnification  of  the  mesial  marginal  ridge,  shown  in  Fig.  105. 
(About  60  X) 


Fig.  107 


Occlusal  and  mesial  views  of  a  superior  bicuspid,  showing  position  of  section. 
A  beginning  caries  could  be  seen  on  the  surface,  but  it  does  not  show  well  in  the 
picture.     The  section  from  the  buccal  piece  is  shown  in  the  following  illustrations. 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS     139 

of  it.  The  enamel  rods  have  fallen  out  of  the  disintegrated 
area,  and  the  decalcification  in  the  dentine  is  shown  (Fig.  109). 
If  this  had  been  treated  as  a  simple  cavity  the  occlusal  wall 
would  have  required  an  inclination  of  1 8  centigrades  occlusally 
from  the  horizontal  plane  to  reach  the  enamel  rod  direction. 

Fig.  108 


The  section  ground  froia  the  buccal  piece,  1 


There  is  very  little  support  offered  by  the  dentine  for  the 
enamel  of  the  marginal  ridge,  and  the  portion  over  to 
the  occlusal  groove  would  be  likely  to  be  broken  off  by  the 
force  of  mastication.  The  conditions  of  the  occlusal  wall 
are  better  shown  in  Fig.  110. 


140      AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 

Any  number  of  illustrations  of  these  conditions  might 
be  made,  but  the  subject  may  be  summed  up  by  saying: 

Fig.  109 


The  region  of  the   carious  spot  shown  in  Fig.  107,  showing  the  disintegrated  area 
of  the  enamel  and  the  action  of  acid  on  the  dentine.     (About  30  x) 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS     141 


Fig.  no 


The  enamel  over  the  mesial  marscinal  ridge  to  the  oblique  groove,  showing  a  region 
of  weakness  for  the  occlusal  wall  of  a  simple  proximal  cavity. 


142      AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 

The  surface  of  the  enamel  from  the  point  directly  over  the 
dentine  cusp  or  ridge  to  the  tip  of  the  enamel  cusp  or  ridge, 
which  is  an  area  of  great  strength  in  the  perfect  crown,  is  a 
region  of  weakness  for  an  enamel  wall.  It  is  fully  as  impor- 
tant not  to  extend  into  this  area  unnecessarily  as  to  form 
the  wall  properly  when  caries  has  extended  so  as  to  involve 
it.  And  when  caries  of  a  smooth  surface  approaches  a  mar- 
ginal ridge  which  receives  the  force  of  occlusion,  the  wall 
must  be  extended  so  that  the  enamel  receives  full  support 
from  sound  dentine. 


CHAPTER  XII 

THE  EFFECT  OF  CARIES  ON  THE  STRUCTURE  OF  THE 

ENAMEL 

The  action  of  acid  upon  the  enamel  has  been  fully  con- 
sidered in  Chapter  VIII,  and  it  should  be  carefully  studied 
before  considering  the  effect  of  caries  on  the  structure  of 
the  enamel,  for  this  cannot  be  understood  unless  the  relative 
solubility  of  the  rods  and  cementing  substance  and  the 
relationship  of  the  two  structural  elements  are  clearly  in 
mind. 

During  the  last  ten  years  there  has  been  a  great  increase 
in  knowledge  of  the  beginning  of  caries  of  the  enamel  and 
the  extent  of  tissue  injury  before  an  actual  cavity  is  pro- 
duced. This  has  placed  a  tremendous  emphasis  upon  the 
value,  for  the  preservation  of  the  teeth,  of  the  treatment 
of  caries  in  its  early  rather  than  in  its  later  stages.  It  is 
safe  to  say  that  if  caries  progresses  until  a  patient  is  aware 
of  a  cavity,  the  tooth  has  been  injured  more  than  is  necessary 
in  the  most  radical  treatment  of  the  same  cavity  in  its 
beginning  stages.  One  who  has  not  studied  carefully  the 
effect  of  caries  on  the  structure  of  the  enamel,  so  as  to 
recognize  the  extent  of  injury  to  the  structure  of  the  tissue 
by  its  appearance  to  the  naked  eye,  can  never  be  considered 
fit  to  prepare  cavities  as  a  treatment  for  the  disease.  The 
beginnings  of  caries  must  be  divided  into  two  classes:  (1) 
Those  occurring  in  natural  defects  of  structure;  (2)  those 
beginning  upon  smooth  surfaces. 

Caries  Beginning  in  Natural  Defects  of  Structure. — These  are 
the  positions  in  which  caries  first  appears  and  in  which 
it  presents  the  greatest  intensity,  because  they  offer  ideal 
conditions.  Such  open  grooves  and  imperfectly  closed  pits 
in  the  enamel  as  have  been  illustrated  in  Chapter  XI  become 


144      THE  EFFECT  OF  CARIES  ON  THE  ENAMEL 

filled  with  food  debris,  which  furnish  ideal  culture  media  for 
acid-forming  bacteria.  At  the  opening  of  the  defect  the 
acid  is  washed  away  by  the  sahva  as  fast  as  it  is  formed,  but 
at  the  bottom  of  the  groove  it  is  confined  and  acts  upon  the 
enamel,  dissolving  out  the  cementing  substance  from  between 
the  rods  and  following  the  rod  direction  toward  the  dento- 
enamel  junction.  The  form  of  the  disintegrated  tissue  in 
such  positions  is  always  that  of  a  cone  or  wedge,  with  the 
apex  at  the  opening  of  the  pit  or  groove  and  the  base  toward 
the  dento-enamel  junction.  The  formation  of  acid  in  these 
positions  is  often  so  rapid  and  the  confinement  so  perfect 
that  the  carious  process  here  manifests  its  greatest  intensity, 
the  action  often  dissolving  the  rods  as  well  as  the  cementing 
substance  and  progressing  across  the  rods.  But  even  when 
the  action  follows  the  rod  direction,  the  form  will  be  broader 
toward  the  dentine,  as  the  rods  are  inclined  toward  the 
defect.  Figs.  Ill  and  112  show  split  teeth  illustrating  the 
disintegration  of  the  enamel  around  occlusal  defects.  The 
disintegration  area  appears  white  by  reflected  light  because 
the  cementing  substance  has  been  removed  from  between 
the  rods  and  the  resulting  air  spaces  refract  the  light.  As 
soon  as  this  disintegration  reaches  the  dento-enamel  junc- 
tion, the  acid  formed  passes  through  the  now  porous  enamel 
and  acts  much  more  rapidly  upon  the  dentine.  Because  of 
the  branching  of  the  dentinal  tubules  at  the  dento-enamel 
junction,  the  action  upon  the  dentine  spreads  rapidl}^  along 
this  line.  Soon  some  of  the  loosened  rods  be  ween  the  bottom 
of  the  defect  and  the  dentine  are  either  entirely  dissolved 
or  displaced  or  dislodged,  and  the  microorganisms  are 
admitted  to  the  dentine.  The  decalcified  dentine  matrix 
becomes  food  material  for  the  bacteria,  and  the  space  pro- 
duced by  the  destruction  of  tissue  furnishes  greater  space 
for  decomposing  foodstufi^s.  The  acids  formed  attack  the 
enamel  from  within  outward,  producing  what  has  been 
called  backward  or  secondary  decay  of  enamel.  At  the  mouth 
of  the  defect  the  acid  is  still  washed  away,  and  there  is 
little  action  upon  the  tissue.  The  condition  progresses, 
therefore,  until,  as  in  Fig.  113,  the  entire  occlusal  enamel 


THE  EFFECT  OF  CARIES  ON   THE  ENAMEL       145 

has  been  undermined,  and  all  of  the  undermined  area  has 
been  greatly  weakened  by  the  solution  of  the  cementing  sub- 

Fia.  Ill 


A  split  tooth,  showing  caries  beginning  in  an  occlusal  groove. 


Fig.  112 


A  split  tooth,  showing  caries  progressing  in  an  occlusal  groove. 


10 


146       THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 

stance  from  between  the  rods.  In  general  sections  of  such 
areas  as  shown  in  Fig.  116  the  disintegrated  area  appears 
dark  by  transmitted  Hght.  Fig.  114  shows  the  progress  of 
secondary  decay  from  an  occhisal  cavity.  In  this  way  it 
often  happens  that  the  entire  occlusal  enamel  is  destroyed 
before  the  original  defect  is  noticeably  enlarged. 

The  general  form  of  the  disintegrated  area  in  caries 
beginning  in  natural  defects  may  be  described  diagram- 
maticall}',  as  in  the  enamel  a  cone  or  wedge  with  the  apex 
toward  the  mouth  of  the  defect  and  the  base  toward  the 
dento-enamel  junction,  and  in  the  dentine  a  cone  or  wedge 
with  the  base  at  the  dento-enamel  junction  and  the  apex 
toward  the  pulp. 

Fig.  113 


A  split  tooth,  showing  the  undermining  of  the  occlusal  enamel  by  caries  spreading 
at  the  dento-enamel  junction. 


Caries  Beginning  on  Smooth  Surfaces. — Caries  upon  smooth 
surfaces  of  the  enamel  is  always  due  to  the  growth  of  a 
colony  of  bacteria  which  becomes  attached  to  the  surface 
by  the  formation  of  material,  causing  them  to  adhere  to  the 
surface  and  at  the  same  time  confining  their  acid  products 
in  contact  with  the  enamel  preventing  its  dissipation  in 
the  saliva  and   allowing  it  to  combine  with  the  inorganic 


CARIES  BEGINNING  ON  SMOOTH  SURFACES     147 

salts  of  the  tissue  elements.  This  is  not  the  place  to  con- 
sider the  bacteriology  of  caries,  but  the  effect  upon  the 
structure  of  the  enamel  cannot  be  understood  without  a 

Fig.  114 


A  section  shoTving  the  undermining  of  the  enamel  and  secondary  or  backward 
decay  at  1. 


clear  conception  of  the  microbic  plaques.  A  growth  of 
masses  of  microorganisms  upon  the  surface  of  a  tooth  does 
not  constitute  a  plaque.  ]Many  very  filthy  mouths  are 
found  where  most  of  the  surfaces  of  the  teeth  are  covered 


148       THE  EFFECT  OF  CARIES  ON  THE  ENAMEL 

by  thick,  furry  masses,  and  where  there  is  little  or  no  attack 
of  the  enamel.  Either  acid  is  not  formed  or  it  is  at  once  lost 
by  solution  in  the  saliva.  Caries  shows  the  greatest  intensity 
in  comparatively  clean  mouths,  in  which  something  in  the 
nature  of  the  saliva  causes  the  bacteria  to  produce  a  tough 
zooglea,  which  attaches  them  to  the  tooth  surface  and  con- 
fines the  products  of  their  activity.  This  zooglea  presents 
some  of  the  phenomena  of  a  dialyzing  membrane.  Through 
it  the  microorganisms  receive  their  food  materials,  and  their 
products  are  neutralized  by  chemical  action  on  the  surface 
upon  which  the  colony  is  growing.  Colonies  lodge  in  the 
most  favorable  spots  and  extend  from  these  points  into 
areas  that  are  less  liable  to  maintain  their  attachment.  The 
more  perfect  the  confinement  of  the  acid,  and  the  more 
rapid  the  rate  of  its  formation,  the  greater  w^ill  be  the  intensity 
of  the  destructive  process.  The  more  easily  the  colony  is 
able  to  maintain  itself  in  its  position  and  extend  upon  the 
surface,  the  greater  is  the  liability.  As  the  colony  becomes 
thickest  at  the  point  of  beginning,  it  is  evident  that  the  most 
acid  is  formed  here,  and  it  is  therefore  the  point  of  greatest 
intensity.  It  is  also  the  point  at  which  the  growth  began, 
and  therefore  the  spot  where  the  action  on  the  tissue  has 
been  longest  in  operation.  It  is  also  apparent  that  there  may 
be  great  intensity  with  limited  liability,  and  great  liability 
with  very  low  intensity,  and  the  effect  upon  the  tissue  will 
be  different  in  the  two  cases. 

The  appearance  of  the  tissue  becomes  an  index  for  esti- 
mating the  intensity  and  liability  in  a  given  case.  The  char- 
acter of  the  effect  of  the  disease  on  the  appearance  of  the 
enamel,  as  well  as  the  direction  of  the  extension  upon  the 
surface  of  the  tooth,  become  most  important  factors  in 
the  diagnosis  of  any  case,  and  the  diagnosis  is  the  basis  for 
the  treatment  required.  The  increased  appreciation  of  the 
extent  of  disintegration  of  the  enamel  before  an  actual 
cavity  is  apparent  in  a  tooth  has  been  one  of  the  most 
important  results  of  Dr.  Black's  study  of  caries  of  the  enamel 
in  the  last  ten  years.  The  author  has  been  intimately 
associated  with  this  work,  and  has  been  amazed  at  the  extent 


PROGRESS  OF  CARIES 


149 


and  character  of  the  effect  of  caries  upon  the  structure  of  the 
enamel  in  what  may  be  called  the  early  stages  of  the  disease. 
Progress  of  Caries. — A  colony  of  bacteria  becomes  attached 
to  the  proximal  surface  of  an  incisor  just  to  the  gingival  of 
the  contact  point,  and  remains  there  some  time.  If  the 
surface  of  the  tooth  can  then  be  examined,  a  white  spot  will 
be  seen  at  Fig.  115;  the  area  appears  white  because  the 
cementing  substance  has  been  removed  from  between  the 
enamel  rods,  as  will  be  seen  later,  and  the  air  that  occupies 
the  spaces  diffuses  the  light.  If  a  tooth  is  split  through 
such  a  spot  and  viewed  from  the  surface,  the  appearance 


Fig.  115 


Fig.  116 


A  superior  central  incisor,  showing  a 
white  spot  just  to  the  gingival  of  the 
contact  point. 


A  split  tooth  cut  through  such   a  white 
spot  as  is  shown  in  Fig.  115. 


will  be  as  shown  in  Fig.  116.  If  a  section  were  ground  through 
the  spot  and  the  tissue  preserved,  the  ends  of  the  enamel 
rods  would  be  seen  pointed  and  projecting  like  the  pickets 
of  a  fence,  giving  the  same  appearance  as  that  produced  by 
the  action  of  acid  upon  a  ground  section,  as  illustrated  in 
Fig.  16,  Chapter  IV.  The  surface  of  the  enamel  is  therefore 
longer  smooth,   but  roughened.     The  roughness  may 


no 


often  be  felt  by  passing  a  very  fine  pointed  steel  explorer 
over  the  surface.  If  the  colony  be  dislodged  at  this  stage  it 
is  evident  that  it  is  much  easier  for  a  new  one  to  become 
attached.     These  whitened  areas  are  often  invisible  unless 


150       THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 

the  tissue  is  dried,  because  the  saliva  fills  the  spaces.  If  the 
surface  is  dried  the  refraction  of  the  light  by  the  air  whitens 
the  affected  area. 

A  good  comparison  is  furnished  in  a  very  familiar  phenom- 
enon. Snow  is  white  because  the  air  and  the  microscopic  ice 
crystals  are  of  different  refracting  index,  and  the  light  is 
diffused  by  passing  from  air  and  ice  crystals.  If  a  snowball 
is  saturated  with  water  it  loses  its  whiteness  and  becomes 
translucent,  because  the  water,  which  is  nearly  of  the  same 
refracting  index  as  ice,  fills  the  spaces  between  the  ice  crystals, 
and  the  light  is  not  diffused.  If  the  white  area  of  such  a 
tooth  is  split  through  the  centre  with  an  aluminum  disk 
charged  with  emery  powder,  the  enamel  rods  will  be  found 
entirely  separated  by  the  solution  of  the  cementing  sub- 
stance, and  the  cross-striation  will  be  much  more  apparent 
because  the  unevenness  in  the  diameter  of  the  rods  has  been 
increased  by  the  action  of  the  acid. 

Formerly  it  was  impossible  to  grind  a  section  through  such 
a  spot  and  preserve  the  tissue.  In  1902  the  author  ground, 
by  the  old  hand  methods,  a  large  number  of  sections  for  the 
study  of  enamel  rod  directions.  Fig.  107,  Chapter  XI,  shows 
the  mesial  surface  of  a  bicuspid  split  for  sectioning.  There 
was  a  white  spot  in  the  region  of  the  contact  point  that  can 
scarcely  be  seen  in  the  photograph.  When  the  central 
section  was  ground  and  mounted  (Fig.  108,  Chapter  XI), 
it  was  seen  that  the  enamel  was  disintegrated  through  its 
entire  thickness  and  the  acid  had  affected  the  dentine,  and 
all  the  enamel  rods  were  lost  from  the  disintegrated  area. 
Until  methods  were  devised  by  Dr.  Black,  it  was  impossible 
to  preserve  the  tissue  and  examine  its  condition.  These 
methods  demonstrate  definitely  that  in  the  disintegrated 
area  the  cementing  substance  is  dissolved  in  large  areas 
before  any  of  the  rods  are  dissolved  or  destroyed.  The 
first  sections  of  such  areas  were  obtained  by  polishing  the 
surfaces  and  cementing  the  split  tooth  to  the  cover-glass  with 
balsam,  completing  the  grinding  and  mounting  without 
loosening  the  section.  In  this  way  the  spaces  between  the 
rods  were  filled  with  balsam  and  so  were  held  in  place. 


PROGRESS  OF  CARIES 


151 


Fig.  117  shows  a  photograph  of  a  section  made  in  this  way, 
and  the  spaces  between  the  rods  and  the  distinct  cross-stria- 
tion  are  seen.     Later  it  was  fonnd  that  by  dehydrating  and 


Fig.  11^ 


A  thin  section  of  carious  enamel  ground  on  the  cover-glass  with  balsam:  E,  sound 
enamel;  X,  carious  enamel  in  wliich  the  cementing  substance  had  been  dissolved 
from  between  the  rods. 

immersing  in  a  solution  of  brown  shellac,  the  shellac  could 
be  made  to  take  the  place  of  the  lost  cementing  substance, 
then  the  polished  surface  of  the  sawed-out  section  could  be 
fastened  to  the  cover-glass  with  shellac,  and  the  specimen 


152      THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 

handled   more   easily.     Fig.    118   shows   a   photograph   of 
carious  enamel  made  in  this  way.    The  rods  are  preserved 

Fig.  118 


Carious  enamel  ground  on  the  cover-glass  by  the  shellac  method.  In  the  region 
X  the  cementing  substance  dissolved  from  between  the  rods  has  been  replaced  by- 
shellac. 


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STAGES  IN   THE  PROGRESS  OF  CARIES  153 

in  place  and  the  dark  shellac  marks  the  disintegrated  area 
very  clearly. 

Stages  in  the  Progress  of  Caries. — The  progress  of  caries 
on  smooth  surfaces  of  the  enamel  may  be  divided  in  three 
periods,  according  to  its  effect  upon  the  structure  of  the 
tissue. 

1.  From  the  lodgement  of  the  colony  until  the  action 
reaches  the  dento-enamel  junction. 

2.  From  the  reaching  of  the  dento-enamel  junction  until 
the  rods  begin  to  fall  out. 

3.  After  a  cavity  is  produced. 

First  Period. — The  form  of  the  disintegrated  tissue  in  the 
first  period  is  always  that  of  an  irregular  cone.  Its  base  is 
on  the  surface  of  the  enamel,  its  outline  is  the  boundary 
of  the  colony,  and  the  apex  is  toward  the  dentine  in  the 
direction  of  the  enamel  rods  from  the  starting  point  of  the 
colony.  The  inner  boundary  of  the  area  is  never  even,  but 
shows  flame-like  extensions  toward  the  dentine  in  the 
direction  of  the  rods.  This  is  more  marked  in  some  cases 
than  in  others,  and  sometimes  suggests  that  the  presence 
of  a  colonv  on  the  surface  has  been  intermittent  (Plates 
V,  YI,  VII). 

The  boundary  between  the  perfect  and  the  disintegrated 
area  is  usually  marked  by  a  darker  area,  the  significance 
of  which  is  not  now  understood.  If  the  disease  progresses 
continuously  the  affected  tissue  always  appears  white  by 
reflected  light,  but  if  the  progress  has  been  intermittent, 
especially  if  there  have  been  considerable  periods  in  which 
no  colony  has  been  attached  to  the  surface,  the  area  darkens, 
becoming  brownish  or  almost  black.  This  is  produced  by 
organic  materials  filling  the  space  between  the  enamel  rods 
and  decomposing,  with  the  probable  formation  of  sulphides 
of  dark  color  in  the  spaces.  If  immunity  to  caries  is  attained 
before  the  effect  upon  the  tissue  has  penetrated  to  the  dento- 
enamel  junction,  this  will  occur,  and  the  spot  changes  from 
a  white  to  a  brownish  or  black  color.  Such  spots  will  be 
found  in  some  places  on  most  teeth  extracted  from  immune 
persons.     Work  of  Dr.  Miller  has  indicated  that  such  spots 


FiQ.  119 


A  section  through  a  white  spot  in  the  first  period  of  attack:    X,  disintegrated 
enamel;  E,  sound  enamel;  D,  dentine. 


STAGES  IN  THE  PROGRESS  OF  CARIES 


155 


are  more  resistant  to  the  progress  of  caries  than  perfect 
enamel  surfaces.  At  any  time  during  the  first  period,  there- 
fore, the    destruction   may  be  arrested   by  the  coming  of 

Fig.  120 


A  section  through  a  carious  spot  in  the  first  period.     The  attack  has  apparently  been 
slow  and  intermittent:  X,  disintegrated  enamel;  E,  sound  enamel;  D,  dentine. 


Fig.  121 


X  i. 


\ 


A  section  through  a  carious  spot  in  the  first  period,  showing  the  flame-like  projections  j 

toward  the  dentine:    X,  disintegrated  enamel;  E,  soimd  enamel;  D,  dentine.  | 


STAGES  IN  THE  PROGRESS  OF  CARIES  157 

immunity,  which  prevents  the  attachment  of    colonies  to 
the  tooth  surface  by  the  formation  of  plaques. 

'  Second  Period — This  period  extends  from  the  time  when 
the  action  of  the  acid  reaches  the  dento-enamel  junction 
until  the  rods  are  destroyed  or  fall  out.  As  soon  as  the 
solution  of  the  cementing  substance  reaches  the  dento- 
enamel  junction  at  the  point  of  the  advancing  cone,  the 
solution  of  the  inorganic  salts  from  the  dentine  matrix 
begins.  It  must  be  remembered  that  the  acid  is  formed 
by  the  microorganisms  on  the  surface  of  the  enamel,  and 
filters  through  the  spaces  between  the  enamel  rods.  The 
decalcification  of  the  dentine  may  be  considerable,  while 
the  surface  of  the  enamel  is  still  preserved.  In  this  period 
the  swelhng  of  the  surface  is  always  noticeable.  This  results 
in  increasing  the  area  of  the  contact  and  therefore  allowing 
the  colony  to  extend  its  limits,  increasing  the  extent  of  the 
surface  attack.  This  is  especially  noticeable  toward  the 
gingival,  and  is  shown  in  Plate  V,  which  is,  however, 
shown  in  the  first  period  of  caries.  In  the  disintegrated 
area  in  this  stage,  as  well  as  in  the  first  stage,  the  diameter  of 
the  enamel  rods  is  always  considerably  reduced  and  the 
striation  rendered  more  apparent.  In  caries  of  great  inten- 
sity but  low  liability  the  reduction  in  the  diameter  of  the 
enamel  rods  is  rapid,  and  they  are  soon  destroyed,  while 
the  area  of  the  surface  attacked  is  small  (Fig.  122). 

In  caries  of  low  intensity  but  great  liability  the  diameter 
of  the  rods  is  slowly  reduced,  while  the  area  of  surface 
attacked,  and  consequently  the  area  of  disintegration,  is 
large  (Fig.  123).  These  conditions  should  be  studied  in  the 
macroscopic  appearance  of  caries  at  the  chair. 

The  decalcified  dentine  matrix  shrinks  and  more  or  less 
of  a  space  is  formed  under  the  enamel. 

The  action  of  the  acid  follows  the  tubules  of  the  dentine 
toward  the  pulp,  and  spreads  through  their  branches  later- 
ally near  the  dento-enamel  junction  so  that  the  form  of  the 
disintegrated  dentine  is  always  that  of  a  cone,  with  the  base 
at  the  dento-enamel  junction  and  the  apex  toward  the  pulp 
chamber.     It  is  important,  however,  to  remember  that  in 


158       THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 


Fig.  122 


A  tooth  split  through  a  spot,  showing  great  intensity  but  low  liability 


Fig.  123 


A  tooth  split  through  spots,  showing  low  intensity  but  great  liability. 


STAGES  IX   THE  PROGRESS  OF  CARIES 


159 


this  stage  no  microorganisms  have  entered  the  tissue,  and 
the  effect  upon  it  is  the  result  of  the  action  of  substances 
formed  upon  the  surface.  The  extent  of  enamel  disintegra- 
tion and  decalcification  of  dentine,  in  this  stage,  is  much 
greater  than  anyone  supposed  before  such  specimens  as  the 
present  illustrations  were  made. 


Fig.  124 


A  drawing  showing  the  microorganisms  of  caries  growing  through  the  dentins 
tubules.     (G.  V.  Black.) 


Third  Period. — This  embraces  the  period  after  the  enamel 
rods  have  begun  to  fall  out  and  an  actual  cavity  is  apparent. 
As  soon  as  this  occurs  the  surface  of  the  tooth  at  the  point 
where  the  formation  of  the  colony  began  is  destroyed  and 
the  protected  point  is  lost,  and  the  extension  of  surface 
attack  ceases.  The  microorganisms  are  admitted  to  the  den- 
tine, where  they  grow  through  the  dentinal  tubules,  spreading 
rapidly  at  the  dento-enamel  junction  (Fig.  124).  The  dentine 
is  always  decalcified  in  advance  of  the  penetration  of  the 
microorganisms.    The  acid  formed  within  the  cavity  attacks 


160       THE  EFFECT  OF  CARIES  ON  THE  ENAMEL 

the  cementing  substance  between  the  enamel  rods,  and 
proceeding  from  the  dento-enamel  junction  outward.  This 
is  called  secondary  or  backward  decay  of  the  enamel,  and  as 
a  result  of  it,  large  areas  are  disintegrated  until  they  are 
sufficiently  weakened  to  break  into  the  cavity.  This  con- 
dition is  shown  in  Fig.  1 14,  in  which  the  area  indicated  by  A 
has  had  the  cementing  substance  entirely  removed  from 
between  the  rods,  and  is  in  the  same  structural  condition  as 
the  disintegrated  areas  in  the  first  and  second  stage.  It  is 
safe  to  say  that  in  the  past  few  cavities  have  been  filled 
until  the  enamel  has  caved  in.  It  is  equally  certain  that  in  a 
large  proportion  of  cases,  by  the  time  this  has  happened, 
the  removal  of  all  disintegrated  tissue  will  require  a  greater 
loss  of  tooth  substance  than  would  be  required  for  the 
prevention  of  a  new  surface  attack,  at  the  margin  of  the 
filling,  if  the  case  had  been  treated  as  a  beginning  instead 
of  a  burrowing  decay. 

ATROPHY 

Atrophy  is  a  disturbance  in  the  structure  of  the  enamel 
caused  by  an  arrest  or  perversion  of  the  function  of  the 
enamel-forming  tissue  during  development.  It  may  be 
caused  by  any  diseased  condition  which  is  serious  enough  to 
produce  marked  disturbance  of  nutrition,  but  it  is  especially 
liable  to  follow  infectious  diseases  that  affect  the  epithelium, 
such  as  scarlet  fever,  measles,  etc.  There  are  all  grades  of 
manifestation  of  this  condition,  from  a  slight  disturbance 
in  the  perfection  of  structure  to  complete  loss  of  a  portion 
of  the  tissue.  In  all  cases  the  portion  of  the  tissue  which  was 
being  formed  at  the  time  of  the  disturbance  shows  a  modifi- 
cation of  structure.  The  imperfect  tissue,  therefore,  corre- 
sponds to  the  bands  of  Retzius  and  follows  their  direction, 
not  the  direction  of  the  enamel  rods.  If,  for  instance,  an 
atrophied  groove  shows  upon  the  surface  of  the  enamel  at 
A  in  Fig.  26,  the  disturbance  in  structure  will  not  follow  the 
enamel  rods  to  the  dentine,  but  the  band  of  Retzius,  and  will 
reach  the  dento-enamel  junction  at  B. 


ATROPHY  161 

Character  of  the  Effect  on  the  Tissue. — In  atrophy  the  for- 
mation of  the  rods  is  first  affected,  the  cementing  substance 
becoming  greater  in  amount.  Conditions  which  produce 
sHght  disturbances  are  marked  simply  by  a  prominent  band 
of  Retzius.  These  are  iUustrated  in  Figs.  125  and  126.  In 
such  sections  the  globules  of  which  the  rods  are  composed 
are  more  imperfectly  fused  and  the  difference  in  the  refracting 
index  between  the  rod  substance  and  the  cementing  substance 
is  greater.  The  cementing  substance  also  seems  to  contain 
actual  pigment.  The  zone  of  dentine  which  was  being  formed 
at  the  same  time  usually  shows  a  zone  of  interglobular  spaces, 
the  character  of  which  will  vary  with  the  character  of  the 
defect  in  the  enamel. 

Fig.  125 


Labial  surface  of  a  central  incisor,  showing  atrophy  grooves.     The  stain  in  the  groove 
makes  it  appear  deeper  than  it  is.    Fig.  126  is  a  part  of  a  section  from  this  tooth. 

In  strongly  marked  cases  it  appears  that  no  enamel  at  all 
is  formed  during  a  longer  or  shorter  period,  and  when  forma- 
tion begins  again  the  portion  that  should  have  been  formed 
is  left  out  and  the  new  formation  is  telescoped  on  to  the  old. 
When  the  pathological  condition  begins  to  affect  the  enamel- 
forming  organ,  the  rods  are  more  and  more  imperfectly 
formed  and  finally  disappear,  cementing  substance  contin- 
uing to  be  deposited.  The  entire  crown,  therefore,  is  short- 
ened and  has  the  characteristic  stunted  appearance.  This 
will  be  understood  by  a  study  of  Figs.  127,  128,  and  129. 
On  the  surface  of  the  tooth,  where  the  new  formation  joins 
11 


162       THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 

the  old,  there  is  a  groove  the  depth  of  which  is  determined 
partly  by  the  duration  of  the  disturbance  and  partly  by  its 
severity. 

The  grooves  of  atrophy  are  usually  accompanied  to  a 
greater  or  less  extent  by  pits  in  which  the  formation  of  rods 
seems  to  have  failed  entirely  and  the  tissue  is  composed  of 
an  imperfect  granular  material  representing  defective  inter- 
prismatic  substance. 

Fig.  126 


injurj'  from  atrophy.  The  growth  of  enamel  -was  interrupted,  but  not  stopped. 
The  zone  of  interglobular  spaces  in  the  dentine  separates  the  dentine  formed  before 
the  interruption  from  that  formed  after.      (Black.) 


White  Spots. — White  spots  are  often  seen  (Fig.  130)  in 
the  enamel  of  one  or  two  teeth.  The  surface  in  these  cases 
is  usually  smooth  and  vitreous  like  the  rest  of  the  tooth. 
Sections  ground  through  such  spots  show  that  in  the  area  of 
the  spot  the  rods  have  been  perfectly  formed,  but  no  cement- 


ATROPHY 


163 


ing  substance.  When  such  an  area  is  entirely  within  the 
substance  of  the  enamel,  as  they  usually  are,  and  the  surface 
of  the  tooth  is  covered  with  normal  tissue,  the  spot  remains 
white,  or  with  the  same  appearance  it  had  when  the  tooth 
erupted.  If  the  defective  structure  reaches  the  surface  they 
may  become  more  and  more  stained. 

Fig.  127 


Section  of  incisors,  showing  two  zones  of  atrophy,  appearing  as  two  grooves  on  the 
surface.      (About  8  X)      (Black.) 


The  spaces  between  the  rods  are  occupied  by  air  and  the 
refraction  diffuses  the  light,  causing  the  white  appearance. 
There  is  no  way  to  remove  these  spots  except  by  polishing 
away  the  tissue,  and  this  is  never  advisable.    When  the  white 


164      THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 

area  reaches  the  surface  of  the  enamel  it  is  not  smooth  and 
poHshed,  as  in  the  normal  condition,  but  rough  and  chalky. 
One  or  two  cases  have  been  reported  in  which  all  of  the 

Fig.  128 


Atrophy:  A  portion  showing  the  incisal  groove  of  Fig.  127  more  highly  magnified. 
The  dark  Une  separates  the  enamel  of  the  first  formation  from  that  of  the  second. 
Notice  that  the  second  is  lapped  on  the  first.  The  narrow  line  of  interglobular 
spaces  is  seen  in  the  dentine.     (Black.) 


Fig.  129 


The  second,  or  gingival,  groove  shown  in  Fig.  127.  The  overlapping  of  the  third 
formation  on  the  second  is  not  as  great,  but  the  discoloration  is  greater.  The  band 
of  interglobular  spaces  in  the  dentine  is  much  wider,     (Black.) 


ATROPHY 


165 


enamel  of  all  the  teeth  was  of  this  character.^  Instead 
of  the  normal  color  the  tissue  was  originally  white  like  a 
sheet  of  paper,  but  it  became  very  much  stained  and  dis- 
colored. It  was  soft  and  chalky  and  could  be  picked  to 
pieces  with  an  instrument.  When  ground  sections  were 
examined  it  was  found  that  the  interprismatic  substance 
was  entirely  absent  and  that  the  rods  were  standing  unsup- 
ported with  spaces  between  them  (Fig.  131). 


Fig.  130 


A  section  through  a  white  spot  in  the  enamel.     In  the  white  area  the  enamel  rods 
are  without  cementing  substance  between  them.     (Black.) 

Mottled  Enamel. — In  certain  restricted  geographical  areas 
there  seems  to  be  a  tendency  to  imperfections  in  the  forma- 
tion of  the  enamel.  In  these  places  the  teeth  of  many 
children  present  when  they  erupt  white  or  mottled  areas. 
There  may  be  only  a  few  spots  on  a  few  teeth,  or  it  may 


1  Black's  Operative  Dentistry,  vol.  i,  p.  35. 


166      THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 


involve  all  of  the  enamel  of  all  the  teeth.  In  some  places 
a  large  proportion  of  all  the  children  born  and  brought  up  in 
the  district  have  teeth  more  or  less  disfigured.  These  mottled 
teeth  seem  to  be  often  accompanied  by  greatly  freckled 
skin. 

Fig.  131 


;^/ 


Enamel  from  a  h^ection  ground  by  Dr.  Black  trom  near  the  cusps  of  one  of  the 
teeth  in  Dr.  Prunty's  case,  showing  enamel  rods  breaking  into  bundles,  which  end 
in  spiculse.  This  enamel  has  no  cementing  substance  between  the  rods.  Its  color 
was  white  like  paper.     (Black.) 

Nothing  is  now  known  as  to  the  cause  of  this  condition  or 
how  the  enamel  organ  is  affected  to  produce  this  result.  The 
study  of  this  enamel  has  shown  that  the  enamel  rods  are 
perfectly  formed,  but  that  the  cementing  substance  is  entirely 
absent  in  the  mottled  area.  When  the  spots  are  white  the 
spaces  between  the  rods  are  entirely  empty;  when  they  are 
brown  or  a  dark  color,  they  are  filled  more  or  less  with  some 
sort  of  coloring  matter.  In  many  cases  there  is  more  or  less 
pigment  in  these  spaces  before  the  teeth  erupt,  and  in  some 
cases  they  grow  darker  with  age. 


CHAPTER  XIII 

THE  DENTINE 

The  dentine  may  be  defined  as  a  connective  tissue  whose 
intercellular  substance  is  calcified.  It  is  apparently  homo- 
geneous in  structure,  but  penetrated  by  minute  canals, 
which  contain  protoplasmic  projections  from  cells  lying 
within  a  cavity  enclosed  by  the  tissue. 

The  Function  of  the  Dentine. — The  dentine  makes  up  the 
mass  of  the  tooth,  giving  to  it  its  general  form,  each  cusp 
and  root  being  indicated  in  it.  It  gives  to  the  tooth  its 
elastic  strength,  and  the  enamel,  being  hard  and  very 
resistant  to  abrasion,  but  extremely  brittle,  is  dependent 
upon  the  elastic  support  of  the  dentine.  This  has  been 
elaborated  to  a  considerable  extent  in  the  chapter  on  the 
Dental  Tissues.  The  fact  that  the  dentine  gives  the  strength 
to  the  tooth  should  never  be  lost  sight  of  in  operating,  and 
sound  dentine  should  never  be  sacrificed  unnecessarily  in 
the  preparation  of  cavities. 

Structural  Elements  of  the  Dentine.— The  structural  ele- 
ments of  the  dentine  may  be  stated  as: 

1.  The  dentine  matrix. 

2.  The  sheaths  of  Newman  and  the  dentinal  tubules. 

3.  The  contents  of  the  dentinal  tubules  or  the  dentinal 
fibrils. 

While  these  are  the  elements  of  which  the  tissue  is  com- 
posed, there  are  other  characteristic  appearances  found  in 
the  dentine,  caused  by  special  conditions  or  arrangement  of 
these  elements  which  must  be  studied.  These  are  the  granu- 
lar layer  of  Tomes,  the  interglobular  spaces  and  the  lines 
of  Schreger,  and  secondary  dentine. 

Origin  of  the  Tissue  (Histogenesis). — The  dentine,  like 
all  of  the  other  calcified  tissues  except  the  enamel,  is  a 


168  THE  DENTINE 

connective  tissue,  and  is  formed  by  the  dental  papilla,  which 
is  a  conical  papilla  of  connective  tissue  rich  in  bloodvessels 
and  covered  on  its  surface  by  the  layer  of  dentine  forming 
cells,  the  odontoblasts.  The  dentine  is  formed  from  without 
inward,  leaving  the  remains  of  the  dental  papilla  in  the 
cavity  of  the  formed  dentine  as  the  dental  pulp.  Before  the 
tooth  is  erupted,  and  up  to  the  time  that  the  full  length  of 
the  root  is  formed,  a  characteristic  thickness  of  dentine  is 
formed,  which  is  called  the  primary  dentirie.  After  this  time 
dentine  is  formed  by  the  pulp  only  intermittently,  in  response 
to  irritations  and  trophic  impulses,  producing  secondary 
dentine.  Secondary  dentine  is  always  more  irregular  in  the 
arrangement  of  the  tubules,  and  more  imperfect  in  structure 
than  the  primary  dentine.  The  boundary  line  between  two 
periods  of  dentine  formation  can  always  be  picked  out  by 
changes  in  the  direction  or  character  of  the  dentinal  tubules. 

The  Dentine  Matrix. — The  dentine  matrix  is  a  solid,  appar- 
ently homogeneous,  and  very  elastic  substance,  through 
which  the  dentinal  tubules  extend.  It  is  translucent  in 
appearance  and  slightly  yellowish  in  color.  In  broken  or 
split  sections  to  the  unaided  eye  it  has  a  yellowish  color 
by  reflected  light,  and  a  characteristic  luster  due  to  the 
refraction  of  light  by  the  tubules.  In  ground  sections,  by 
transmitted  light,  under  the  microscope,  it  is  very  translucent 
and  shows  no  indication  of  structure. 

The  matrix  consists  of  an  organic  basis  of  ultimately 
fibrous  character,  yielding  gelatin  on  boiling,  with  which  the 
inorganic  salts  are  chemically  combined.  The  relation  of 
organic  and  inorganic  matter  in  the  dentine  matrix  is  similar 
to  the  condition  in  the  bone  matrix  and  that  of  all  calcified 
connective  tissues.  Apparently  the  organic  basis  is  first 
formed,  and  then  the  inorganic  salts  are  combined  with  it  in 
a  weak  chemical  union.  If  the  dentine  is  treated  with  dilute 
acid,  the  inorganic  matter  is  dissolved  and  the  organic  basis 
is  left  retaining  the  form  of  the  tissue.  If  the  organic  matter 
is  burned  out,  it  leaves  the  inorganic  matter  in  the  character- 
istic form. 


THE  SHEATHS  OF  NEWMAN  169 

Von  Bibra  gives  the  following  analysis  of  perfectly  dry 
dentine : 

Organic  matter 37.61 

Fat 0.40 

Calcium  phosphate  and  fluoride 66 .  72 

Calcium  carbonate 3.36 

Magnesium  phosphate 1.08 

Other  salts 0.83 

Dr.  Charles  Tomes  pointed  out  that  such  analysis  as 
this  failed  to  take  account  of  about  8  per  cent,  of  water 
which  is  held  as  water  of  combination,  and  which  is  driven 
off  at  about  red  heat. 

It  is  evident  that  the  organic  matter  in  the  dentine  is  of 
two  kinds — the  organic  basis  of  the  matrix,  which  is  of 
gelatin  yielding  character,  and  the  protoplasmic  contents 
of  the  dentinal  tubules.  Variations,  therefore,  in  the  pro- 
portion of  organic  and  inorganic  matter  in  the  dentine  might 
be  caused  by  differences  in  the  proportions  of  organic  and 
inorganic  constituents  of  the  matrix,  or  by  variations  in  the 
size  of  the  tubules  and  the  amount  of  material  contained  in 
them. 

If  dentine  changes  in  its  degree  of  calcification  with  age, 
this  might  be  brought  about  by  the  reduction  in  the  size 
of  the  tubules,  or  by  the  adding  of  inorganic  constituents 
to  the  matrix. 

The  ultimately  fibrous  character  of  the  dentine  matrix 
can  be  made  out  only  in  various  stages  of  decalcification 
and  decomposition.  In  the  original  condition  no  trace  of 
the  fibrous  character  can  be  seen.  By  maceration  with 
acids  and  alkalies  the  intertubular  material  assumes  a 
fibrous  appearance,  as  if  bundles  of  white  connective-tissue 
fibers  had  been  fused  together.  There  is  apparently  no 
definite  arrangement  of  these  fibers  and  there  is  no  indica- 
tion of  the  arrangement  of  the  substance  in  layers. 

The  Sheaths  of  Newman. — There  has  been  much  discussion 
as  to  i:he  character  of  these  structures,  which  were  first 
discovered  in  1863  by  Newman.  Some  investigators  have 
denied  their  existence  entirely,  explaining  the  appearance 


170 


THE  DENTINE 


in  some  other  way.  These  structures  are  in  no  sense  a 
sheath  surrounding  tlie  dentinal  fibril  and  lying  in  the 
dentinal  tubule,  but  are  that  portion  of  the  matrix  which 
forms  the  immediate  wall  of  the  tubule.  That  this  material 
differs  from  that  which  occupies  the  rest  of  the  space  between 
the  tubules  is  certain,  and  is  shown  by  the  examination  of 
ground  sections,  the  action  of  stains  upon  ground  sections, 
and  the  action  of  the  matrix  when  boiled  with  strong  acids 


Fig.  132 


""-•--  .  --^ 

^■^Jtrmi.,   M 

'  ^fsp  >*  ^-~---— ^  ™ 

■j 

1 

^i 

f  c' 

Dt 
D 


Dentine  showing  tubules  in  cross-section:   Dt,  dentinal  tubules;    D,  dentine  matrix; 
S,  shadow  of  sheaths  of  Neumann.     (About  1150  x) 

and  alkalies.  In  Fig.  132,  a  photograph  of  a  ground  section, 
there  is  evidently  a  difference  in  the  refracting  index  of  the 
portion  of  the  matrix  immediately  surrounding  the  tubules. 
Apparently  the  sheaths  of  Newman  are  composed  of  a 
material  similar  to  that  forming  elastic  connective-tissue 
fibers,  and  known  as  elastin.  This  substance  is  very  resistant 
to  the  action  of  acids  and  alkalies.  After  the  remainder 
of  the  intertubular  material  has  been  destroyed  by  boiling 


DIRECTION  OF  TUBULES  IN  CROWN  PORTION     171 

with  strong  acid,  the  sheaths  remain  Uke  hollow  elastic 
fibers,  having  the  appearance  of  pipestems,  which  resist 
long  continued  action  of  the  boiling  acid.  Some  authors 
have  suggested  that  the  great  elasticity  of  the  dentine  was 
largely  due  to  the  presence  of  this  substance. 

The  Dentinal  Tubules. — The  dentine  matrix  is  penetrated 
everywhere  by  minute  branching  tubules,  which  radiate 
from  the  central  cavity  or  pulp  chamber  and  extend  to  the 
outer  surface  of  the  dentine  at  the  dento-enamel  junction  or 
the  dentocemental  junction,  where  they  end  blindly  or  in 
irregular  enlargements.  These  tubules  are  from  1.1  to  3 
microns  in  diameter.  One  hundred  measurements^  made  at 
random  from  ground  sections  gave  the  extreme  measure- 
ment: 3,  largest;  1.5,  smallest;  and  average,  2.95.  Fifty 
measurements  from  one  longitudinal  section  of  tubules  at 
their  pulpal  extremity  gave  an  average  of  2.6;  largest,  3; 
smallest,  1.5;  and  50  measurements  at  the  dento-enamel 
junction  of  the  same  section  gave  the  following:  Average, 
1.2;  largest,  1.5;  smallest,  0.75.  These  measurements  were 
made  with  an  eye-piece  micrometer,  using  yV  oil  immersion 
objective  and  No.  3  ocular. 

At  the  present  time  there  is  a  fertile  field  for  investigation 
offered  in  regard  to  the  size  of  dentinal  tubules.  ]\Iany 
statements  have  been  made  that  have  not  been  supported 
by  tabulated  measurements,  and  no  definite  statement  can 
be  made  as  to  the  variations  and  size  of  the  dentinal  tubules 
in  different  teeth,  the  teeth  of  different  animals,  or  in  the 
human  teeth  at  different  ages. 

Direction  of  Tubules  in  Crown  Portion.  —  In  the  crown 
portion  and  the  gingival  portion  of  the  dentine  the  tubules 
pass  from  the  pulp  chamber  to  the  dento-enamel  junction, 
or  the  dentocemental  junction,  in  sweeping  curves,  which 
were  called  by  Tomes  the  primary  curvatures.  These 
have  been  described  as  /-  or  *S-shaped  (Fig.  133).  The 
tubule  tends  to  enter  the  pulp  chamber  at  right  angles  to 
the  surface,  and  to  end  at  the  dento-enamel  junction  at 

'  KoUiker  gives  5  microns,  also  Schafer;  Owen,  2.5  microns. 


172 


THE  DENTINE 


right  angles  to  that  surface.  In  the  dentine  forming  the 
axial  walls  of  the  pulp  chamber  the  tubules  make  two  bends 
in  passing  from  the  pulp  chamber  to  the  surface  of  the 
dentine.  In  the  first  the  convexity  is  directed  apically,  in 
the  second  it  is  directed  occlusally.  The  outer  extremity  of 
the  tubule  is,  therefore,  considerably  farther  to  the  occlusal 
than  the  point  at  which  it  opens  into  the  pulp  chamber 


Fig.  133 


A  section  showing  tlie  primarj-  curvatures  of  the  dentinal  tubules  in  the  crown 
portion.     (About  20  X) 

(Fig.  134).  The  outer  part  of  this  double  curve  is  often 
complex  instead  of  simple  (Fig.  135).  The  course  of  the 
dentinal  tubules  is  not  a  direct  one,  but  that  of  an  open 
spiral.  This  may  easily  be  demionstrated  by  changing 
the  focus  up  and  down  in  examining  sections  cut  at  right 
angles  to  the  direction  of  the  tubules.  When  examined  in 
longitudinal  sections  this  spiral  course  gives  to  the  tubule 


DIRECTION  OF  TUBULES  IN  CROWN  PORTION     173 

the  appearance  of  having  httle  wavy  curves  throughout  its 
length.  These  have  often  been  called  the  secondary  curva- 
tures. Each  wave  represents  a  turn  in  the  spiral.  As  many 
as  two  hundred  have  been  counted  in  the  length  of  a  single 
tubule,  or  about  one  hundred  in  a  millimeter  of  length. 

Fig.  134 


A  section  showing  the  primary  curvature  of  the  dentinal  tubules  in  the  gingival 
portion.      (About  20  X ) 


The  dentinal  tubules  give  off  minute  lateral  branches, 
which  extend  from  one  tubule  to  another.  These  are  very 
minute,  and  in  the  crown  portion  of  the  dentine  are  not  at 
all   conspicuous,    but   in   the   region   of   the   dento-enamel 


174 


THE  DENTINE 


junction  the  tubules  branch  dichotomously,  each  fork  having 
about  the  same  diameter  as  the  original  tubule  (Fig.  136.) 
These  forkings  of  the  tubules  resemble  the  appearance  of 
the  delta  of  a  river  on  the  map.  The  branches  anastomose 
with  each  other  very  freely.  This  anastomosis  of  the 
tubules  at  the  dento-enamel  junction  is  very  important  in 
determining  the  spreading  of  caries  in  this  area.  It 
probably  also  explains  the  sensitiveness  of  this  area  noticed 
in  the  preparation  of  cavities,  which  will  be  noted  again  in 
considering  the  sensitiveness  of  the  dentine. 


Fig.  135 


A  section  showing  compound  curves  near  the  dento-enamel  junction.    (About  80  X  ) 


'  The  Dentinal  Tubules  in  the  Root  Portion.^In  the  root 
portion  of  the  dentine  the  tubules  ordinarily  show  only  the 
secondary  curves,  their  general  direction  being  at  right  angles 
to  the  axis  of  the  pulp  canal.  Throughout  their  course  they 
give  off  an  enormous  number  of  very  fine  branches  extending 
from  tubule  to  tubule.  These  are  so  numerous  that  in 
suitably  prepared  sections  they  may  be  said  to  look  like  the 
interlacing  twigs  of  a  thicket  or  the  rootlets  of  plants  in  the 
soil.  Fig.  137  gives  a  very  good  idea  of  the  appearance. 
At  the  dentocemental  junction  the  tubules  end  in  irregular 


DENTINAL    TUBULES  IN   THE  ROOT  PORTION     175 

anastomosing  spaces,  which  cause  the  appearance  of  the 
granular  layer  of  Tomes  (Fig.  138). 

From  a  consideration  of  the  preceding  it  will  be  seen  that 
it  is  usually  not  difficult  to  determine  whether  a  field  of 

Fig.  136 


Dentine  at  dento-enamel  junction,  showing  tubules  cut  longitudinally:   Dt,  dentinal 
tubules;   D,  dentine  matrix.     (About  760  X) 

dentine  seen  under  the  microscope  was  taken  from  the  crown 
or  the  root  of  a  tooth.  The  structural  characteristics  of  the 
two  regions  may  be  summarized  as  follows:  In  the  croicn, 
the  tubules  show  both  the  primary  and  the  secondary  curves. 


176 


THE  DENTINE 


In  the  root,  the  tubules  show  only  the  secondary  curves. 
In  the  croum,  the  lateral  branches  are  few  and  inconspicuous 
and  the  tubules  branch  in  a  characteristic  way  at  the  dento- 
enamel  junction.    In  the  root,  the  lateral  branches  are  very 


Fig.  137 


Dentine  from  the  root,  showing  tubules  cut  longitudinally.     (About  700  X ) 


numerous  throughout  the  length  of  the  tubule,  and  they 
end  in  the  characteristic  spaces  of  the  granular  layer  of 
Tomes. 

The   Dentinal  Fibrils. — In   life  the   dentinal   tubules   are 
occupied  by  protoplasmic  projections  of  the  odontoblasts 


THE  DENTINAL  FIBniLS 


177 


known  as  the  dentinal  fibrils,  or  fibers  of  Tomes.  x\s  the 
dentine  matrix  is  formed  and  calcified  under  the  influence 
of  the  odontoblasts,  a  portion  of  their  protoplasm  is  left  in 
the  tubules  of  the   matrix  as  the  dentinal  fibril.     These 


Fig.  138 


Granular  layer  of  Tomes:   L,  lacunae  of   cementum;   Gt,  granular   layer   of    Tomes; 
Ig,  interglobular  spaces.      (About  200  X) 


structures  were  first  described  by  Jolm  Tomes,  who  recog- 
nized tKeir  true  character.  They  may  be  demonstrated 
in  decalcified  sections,  and  they  will  be  seen  projecting  from 
the  odontoblasts,  when  the  pulp  is  removed  from  a  freshly 
12 


178  THE  DENTINE 

extracted  tootli,  by  cracking  it  and  picking  the  pulp  out. 
In  tliis  way  a  portion  of  the  fibril  is  pulled  out  of  the  tubules. 
The  fibrils  will  be  considered  more  especially  in  connection 
with  the  pulp,  to  which  they  properly  belong. 

The  Granular  Layer  of  Tomes. — The  granular  laj^er  of 
Tomes  is  the  outer  layer  of  the  dentine  next  to  the  cementum. 
The  granular  appearance  is  caused  by  irregular  spaces  in 
the  dentine  matrix  which  connect  with  the  ends  of  the 
dentinal  tubules,  and  wdiich  are  filled  with  protoplasm 
continuous  with  that  of  the  fibrils.  Tomes  first  called 
attention  to  this  layer,  and  for  this  reason  it  bears  his  name. 

With  magnifications  of  from  50  to  100  diameter  it  is  easily 
seen  in  ground  sections  either  longitudinal  or  transverse,  and 
appears  as  a  layer  filled  with  little  dark  spots  or  granules, 
the  spaces  which  have  been  filled  with  the  debris  of  grinding. 
It  is  separated  from  the  cementum  by  a  thin  clear  layer, 
apparently  of  structureless  dentine  matrix,  which  is  more 
apparent  in  higher  magnifications.  The  granular  layer  is 
sometimes  seen  in  the  crown  portion  just  under  the  enamel, 
but  it  is  never  as  well  marked  in  this  position. 

The  layer  is  seen  in  sections  ground  from  freshly  extracted 
teeth  as  w^ell  as  from  old  dry  teeth,  showing  that  these  are 
true  spaces  and  are  not  produced  by  the  shrinkage  of  par- 
tially calcified  dentine  matrix.  Tomes  called  the  spaces  in 
the  granular  layer  "interglobular  spaces,"  but  this  term 
should  not  be  used,  as  the  structures  generally  known  as 
the  interglobular  spaces  are  different  in  location  and  char- 
acter, and  will  be  considered  later. 

The  granular  layer  is  not  seen  in  decalcified  sections.  So 
far  as  the  author  is  aware,  no  one  has  called  attention  to  this 
fact  before.  In  decalcified  sections  stained  with  hematoxylin 
and  eosin  the  position  of  the  granular  layer  is  always  occu- 
pied by  a  clear  layer  which  takes  the  stain  in  an  entirely 
different  way  from  the  rest  of  the  dentine  matrix,  and  in 
which  no  indication  of  spaces  can  be  seen.  While  the 
fibrils  in  the  tubules  through  most  of  the  dentine  take  the 
hematoxylin  stain  and  can  be  easily  seen,  they  cannot  be 


THE  INTERGLOBULAR  SPACES  179 

followed  into  this  clear  layer,  and  no  indication  of  proto- 
plasmic contents  of  irregular  spaces  can  be  seen.^ 

Most  authorities  state  that  the  spaces  of  the  granular 
layer  communicate  with  the  canaliculi  of  the  cementum, 
as  well  as  with  the  tubules  of  the  dentine.  This  the  author 
has  been  unable  to  confirm.  On  the  other  hand,  the  granular 
layer  seems  to  be  separated  from  the  cementum  by  a  thin 
layer  of  dentine  which  is  clear  and  apparently  structureless. 
This  is  separated  from  the  cementum  by  a  dark  line,  and 
the  first  layer  of  cementum  usually  does  not  contain  any 
lacunae  or  canaliculi.  This  is  supported  by  some  of  the 
experiments  that  have  been  made  with  extracted  teeth.  In 
experimenting  on  the  diffusion  of  drugs  through  dentine, 
it  was  found  that  liquids  sealed  in  the  pulp  chambers  of 
extracted  teeth  could  not  be  detected  in  the  liquids  in  which 
the  teeth  were  placed  unless  the  cementum  was  removed  from 
them.  In  the  recent  experiments  of  Dr.  Southwell,  of 
Milwaukee,  in  which  air  was  forced  through  the  dentine 
from  the  pulp  chamber,  to  test  the  sealing  of  cavities  by 
filling  materials,  the  air  did  not  escape  from  the  cementum, 
which  would  be  the  case  if  dentinal  tubules  connected  with 
the  canaliculi  of  the  cementum. 

If  the  spaces  of  the  granular  layer  are  filled  with  the  pro- 
toplasmic enlargements  of  the  ends  of  the  dentinal  fibrils, 
this  would  give  a  very  reasonable  explanation  of  the  sensi- 
tiveness of  slight  caries  and  erosion  at  the  gingival  line,  as 
the  anastomosis  through  the  granular  layer  would  affect 
the  fibrils  of  the  entire  root. 

The  Interglobular  Spaces. — There  has  been  considerable 
misunderstanding  in  dental  histology  in  regard  to  these 
spaces,  owing  to  the  confusing  of  two  entirely  different 
things.  Tomes  called  the  spaces  of  the  granular  layer, 
which  have  already  been  described,  interglobular  spaces. 
As  has  been  seen,  they  are  true  spaces  in  the  dentine  matrix 


1  The  appearance  of  the  tissue  in  decalcified  sections  has  led  to  some  doubt  in  the 
writer's  mind  as  to  the  interpretation  of  the  character  of  the  laj-er  by  authors  who 
have  described  it. 


ISO 


THE  DENTINE 


which  connect  with  the  dentinal  tubules  and  are  filled  with 
protoplasm. 

In  1850  J.  Czermak^  described  areas  of  imperfectly  cal- 
cified dentine  matrix,  which  appear  as  spaces  in  dried  dentine, 
and  called  them  interglobular  spaces.  These  have  been  so 
called  by  most  writers  since.  It  seems  important  to  the 
author  that  the  term  be  restricted  to  these  and  some  others 
used  to  indicate  the  spaces  of  the  granular  layer,  which  are 
of  entirely  different  character. 


Fig.  139 


A  drawing  showing  a  zone  of  interglobular  spaces  in  the  dentine.     (Black.) 

The  interglobular  spaces  of  Czermak  are  caused  by  some 
disturbance  in  the  calcification  of  the  organic  matrix  of  the 
dentine.  They  occur  in  zones  (Fig.  1.39)  which  correspond  to 
the  dentine  matrix,  being  calcified  at  a  given  time,  and  there 
is  usually  seen  a  corresponding  disturbance  in  the  calcifica- 


Beitrag  zur  Mikro-Anatomie  der  Menschlichen-Zahne. 


THE  INTERGLOBULAR  SPACES 


181 


tion  of  the  enamel,  which  was  being  formed  at  the  same 
time  and  manifested  as  a  more  or  less  strongly  marked 
atrophy  band. 

Fig.  140 


^im^'^m^- 


^^ 


^-^'^^^^ 


r^^r^^ 


Interglobular  spaces  in  dentine.      (About  60  X ) 


Fig.  141 


Interglobular  spaces  in  dentine.     Some  emptj',  some  filled  wiiL  JlLtis.    Clbout  SO  >,  ' 

In  the  calcification  of  the  dentine  matrix  the  inorganic 
salts  are  combined  with  the  organic  matrix  in  spherical 
areas  which  become  united.  The  boundaries  of  these  areas 
of  uncalcified  matrix  are  therefore  very  irregular,  and  made 


182 


THE  DENTINE 


up  of  concave  facets  where  they  join  the  spherical  surfaces 
of  the  fully  calcified  matrix  (Figs.  140  and  141).  A  study  of 
the  illustrations  and  the  appearance  of  the  layer  of  forming 
dentine  next  to  the  dental  papilla  of  a  developing  tooth  will 
make  this  intelligible. 


Fig.  W2 


Interglobular  spaces  in  dentine:   Ig,  first  line  of  interglobular  spaces;  Ig',  second 
line  of  interglobular  spaces.      (About  30  X ) 

If  the  dentine  is  dried  the  organic  matrix  in  these  areas 
gives  up  water  and  shrinks,  and  the  interglobular  spaces 
become  true  spaces,  partially  filled  with  the  shrunken  matrix. 
In  this  condition  they  can  be  filled  with  colored  collodion 


THE  INTERGLOBULAR  SPACES 


183 


or  any  other  material.  If,  however,  they  are  studied  in 
sections  of  teeth  which  have  never  been  allowed  to  dry,  no 
space  appears,  and  the  dentinal  tubules  continue  without 
change  of  course  or  diameter  through  the  area.  While  they 
are,  therefore,  not  empty  spaces,  they  are  areas  of  the  organic 
matrix  of  the  dentine  which  are  bounded  by  globular  sur- 
faces of  the  fully  calcified  matrix,  and  their  name  is  properly 
significant. 

Fig.  143 


A  root  broken  on  a  line  of  interglobular  spaces.  This  tooth  was  extracted  by 
Dr.  G.  v.  Black,  and  was  pulled  apart  in  extraction,  a  shows  the  form  of  the  root 
and  a  b  the  separation  on  the  line  of  growth.      (Black.) 


Zones  of  interglobular  spaces  may  occur  at  any  portion 
of  the  dentine,  either  in  the  crown  or  root,  but  they  are  more 
common  in  the  crown  and  near  the  enamel.  Often  more  than 
one  zone  can  be  seen,  as  in  Fig.  142,  which  shows  two  dis- 
turbances in  calcification,  and  disturbances  in  the  structure 
of  the  enamel  will  be  seen  at  corresponding  positions. 

The  zones  of  interglobular  spaces  appear  in  all  grades, 
from  a  complete  band  of  uncalcified  matrix  to  widely 
scattered  patches.  Fig.  143  shows  a  tooth  in  Dr.  Black's 
collection  which  was  broken  in  extraction,  because  of  the 
presence  of  such  a  zone  in  the  root. 


184  THE  DENTINE 

The  interglobular  spaces  are  of  great  importance  in  modi- 
fying the  direction  of  the  progress  of  caries  in  the  dentine. 

The  Lines  of  Schreger. — As  in  the  case  of  the  interglobular 
spaces,  there  seems  to  be  considerable  misunderstanding  in 
the  literature,  and  certain  structures  which  have  very 
different  meanings  have  been  called  the  "lines  of  Schreger." 

An  arrest  in  the  formation  of  dentine  often  occurs  before 
the  crown  is  completed.  When  the  activity  has  begun  again 
the  dentinal  tubules  follow  a  slightly  different  direction.  In 
a  longitudinal  section  this  change  in  the  direction  of  the 
tubules  produces  a  line.  Several  such  lines  may  be  seen 
in  a  single  section,  though  they  are  by  no  means  to  be  found 
in  all  longitudinal  sections. 

Schreger's  lines  have  been  most  often  confused  with  zones 
of  interglobular  spaces,  and  they  seem  to  be  identical  with 
the  incremental  lines  in  the  dentine  described  by  Owen. 
It  is  unfortunate  that  these  names  should  have  been  used, 
for  a  thoughtful  study  of  the  tissue  makes  their  interpreta- 
tion perfectly  evident,  and  they  are  of  no  great  significance. 

Secondary  Dentine.— It  is  by  no  means  easy  to  define 
secondary  dentine  or  to  pick  out  any  particular  piece  of 
dentine  in  a  section  and  to  say  whether  it  is  primary  or 
secondary.  In  general,  the  tubules  are  smaller,  fewer,  and 
less  regularly  arranged  in  secondary  than  in  primary  dentine. 
In  general,  it  seems  that  the  smaller  the  remainder  of  the 
dental  papillae  becomes,  the  more  imperfect  dentine  it 
forms,  until  finally  it  simply  throAvs  down  granular  calcified 
material. 

The  formation  of  dentine  begins  at  the  dento-enamel 
junction,  at  a  number  of  points  in  each  tooth,  and  progresses 
from  without  inward  (strange  to  say,  exactly  the  opposite 
statement  has  been  made  several  times  in  papers  by  very 
prominent  men).  This  matter  will  be  taken  up  more  in 
detail  in  Chapters  on  Dental  Embryology  and  Dentition. 
It  is  enough  to  say  here  that  in  studying  all  sections  of 
dentine,  whether  cut  longitudinally  or  transversely,  the 
formation  of  dentine  began  at  the  dento-enamel  junction 
and  the  dentocemental  junction,  and  progressed  toward 
the  pulp  chamber. 


SECONDARY  DENTINE 


185 


From  the  study  of  longitudinal  and  transverse  sections 
it  is  apparent  that  a  certain  typical  amount  of  dentine  is 
formed  before  the  tooth  is  erupted  and  while  it  is  coming 
into  full  occlusion.  This  is  primary  dentine.  In  it  the 
tubules  are  very  regular  in  size  and  arrangement.     From 


Fig.  144 


P 


o^-*^«i^ 


M^AVl 


'•X-^it\»\uMiyiM: 


Secondary    dentine:    A,  margin    of    pnnuay    LkiiUuc  .-Lowiiiy  u  few    ul    thu  tubules 
continuing  into  secondary  dentine;  P,  pulp  chamber.      (About  80 X) 

this  time  on  the  formation  of  dentine  is  intermittent,  and 
apparenth'  is  the  response  to  some  outside  condition.  These 
conditions  may  arise  in  the  tooth  in  which  the  formation 
occurs,  or  the  irritation  of  one  tooth  may  cause  tissue  forma- 
tion in  all  or  part  of  the  others.    It  has  not  been  determined 


186 


THE  DENTINE 


whether  such  reflex  trophic  stimuH  are  confined  to  the  same 
lateral  half  or  the  same  nerve  distribution.  Apparently  the 
formation  of  dentine  proceeds  again,  after  a  pause,  in  all 
teeth.  It  will  seem,  therefore,  that  the  mere  exposure  of 
the  entire  crown  to  conditions  of  thermo-change  produces 
sufficient  stimulus  to  the  pulp  tissue  to  cause  a  renewal  of 
dentine  formation.    After  the  first  period  of  rest  the  dentine 

Fig.  145 


A  transverse  section  of  a  root,  showing  the  reduction  in  the  size  of  the  pulp 
and  formation  of  secondary  dentine. 


formed  in  the  second  period  is  so  nearly  identical,  and  the 
direction  of  the  tubules  so  nearly  the  same,  that  it  is  usually 
impossible  to  recognize  the  junction  except  at  a  few  points 
in  the  circumference  of  a  transverse  section.  After  each 
period  of  rest,  however,  the  difference  in  structure  between 
the  succeeding  portions  becomes  more  marked.     Fig.  144 


SECONDARY  DENTINE 


187 


shows  an  area  from  a  longitudinal  section  when  the  line  A 
was  the  pulpal  wall  of  the  dentine.  There  was  probably 
a  considerable  period  of  rest,  when  for  some  reason  a  new 
formation  of  dentine  was  begun.  But  apparently  only  some 
of  the   odontoblasts  took  part   in  the  new  formation   of 


A  transverse  section  of  a  root,  showing  changes  in  the  form  of  the  pulp  canal 
by  the  formation  of  secondary  dentine. 


dentine  matrix,  for  not  nearly  all  of  the  tubules  are  con- 
tinued, and  those  that  do  continue  show  a  sharp  change  in 
their  direction  and  a  difference  in  diameter  and  character 
(Figs.  145  and  146). 

These  characteristic  changes  in  the  structure  of  the 
dentine  that  is  formed  as  the  pulp  becomes  smaller  seem 
to  the  author  of  great  practical  importance. 


CHAPTER  XIV 

THE  CEMENTUM 

The  cementum  may  be  defined  as  a  connective  tissue  whose 
intercellular  substance  is  calcified  and  arranged  in  layers 
(lamellse)  around  the  circumference  of  a  tooth  root,  the 
cells  being  found  in  spaces  (lacunae)  irregularly  placed  in 
or  between  the  layers. 

Structurally  the  cementum  is  more  closely  related  to  the 
subperiosteal  bone  than  any  other  tissue,  the  only  differences 
being  that  in  general  the  lacunse  in  bone  are  much  more 
uniform  in  size,  shape,  arrangement  of  the  canaliculi,  and 
their  position  with  reference  to  the  lamellse  than  those  in 
cementum.  In  bone  the  lacunae  are  usually  found  between 
the  lamellae.  In  cementum  the  lacunae  may  be  between  the 
lamellae,  but  they  are  more  often  enclosed  within  their  sub- 
stance and  they  are  found  most  often  where  the  lamellae 
are  thick. 

Some  writers  have  described  Haversian  canals  in  the 
cementum,  but  the  author  has  never  seen  anything  that 
could  properly  be  called  an  Haversian  canal  in  the  cementum 
from  human  teeth.  Canals  containing  bloodvessels  are  not 
uncommon,  but  in  these  the  lamellae  are  never  arranged 
concentrically  around  the  canal,  as  they  are  in  Haversian 
systems.  For  the  last  fifteen  years  the  author  has  had  under 
personal  observation,  in  the  course  of  class  work,  not  less 
than  200  longitudinal  sections,  and  300  transverse  sections 
of  the  root,  ground  from  human  teeth,  and  in  that  time  he 
has  never  seen  what  could  be  called  an  Haversian  canal. 
In  the  same  time  he  has  examined  many  hundreds  of 
sections  cut  through  the  decalcified  jaws  of  various  mam- 
mals, including  the  sheep,  pig,  cat,  and  dog,  with  the  same 
negative  result. 


HISTOGENESIS  189 

Function. — The  function  of  the  cementum  is  to  attach  to 
the  tooth  the  connective-tissue  fibers  which  hold  it  in  posi- 
tion and  support  the  surrounding  tissues. 

The  formation  of  cementum  begins  as  soon  as  the  tooth 
begins  to  erupt,  and  continues,  at  least  intermittently,  as 
long  as  the  tooth  remains  in  place,  whether  it  contains  a  live 
pulp  or  not. 

The  function  of  the  cementum  cannot  be  too  strongly 
emphasized,  and  must  be  continually  borne  in  mind.  If,  for 
any  reason,  the  tissues  are  detached  from  the  surface  of  the 
root,  they  can  only  be  reattached  by  the  formation  of  a  new 
layer  of  cementum  on  the  surface  of  the  root,  which  will 
embed  the  surrounding  connective-tissue  fibers.  In  order 
to  accomplish  this  the  tissues  must  lie  in  physiologic  con- 
tact with  the  surface  of  the  root,  and  the  connective-tissue 
cells  must  be  actively  functional. 

That  the  tissues  may  be  reattached  to  the  surface  of  a 
root  is  both  theoretically  possible  and  clinically  demonstrable, 
but  for  it  to  occur,  biological  laws  must  be  observed  and  the 
conditions  are  very  difficult  to  control,  especially  with  the 
old  methods  involving  the  excessive  use  of  strong  antiseptics. 
It  is  well  to  remember  ''that  a  dentist  can  never  cure  a 
suppurating  pocket  along  the  side  of  a  tooth  root,"  but  if 
the  conditions  can  be  controlled  the  cells  of  the  tissue  may 
form  a  new  layer  of  cementum,  reattaching  the  tissues  and 
so  close  the  pocket.  It  is  a  biological  problem,  not  a  matter 
of  drugs,  except  as  they  are  a  means  of  producing  cellular 
reaction. 

In  view  of  its  function,  therefore,  the  cementum  becomes 
not  the  least  but  the  most  important  of  the  dental  tissues,  for 
no  matter  how  perfect  the  crown  may  be,  without  firm 
attachment  the  tooth  becomes  useless  and  is  soon  lost. 

Histogenesis. — The  cementum  is  formed  by  connective- 
tissue  cells  lying  between  the  fibers  of  the  tissue  which  clothes 
the  surface  of  the  root  and  which  becomes  specialized  for 
this  function.  Their  origin  is  undoubtedly  similar  to  that 
of  the  osteoblasts,  but  they  are  not  osteoblasts,  either  mor- 
phologically or  functionally,  as  will  be  seen  later  in  the 


190  THE  CEMENTUM 

study  of  the  peridental  membrane,  where  the  cementoblasts 
and  the  formation  of  cementum  will  be  considered. 

Structural  Elements. — The  structural  elements  of  the 
cementum  are: 

1.  The  lamellae. 

2.  The  lacunse  and  canaHculi. 

3.  The  cement  corpuscles. 

4.  The  embedded  fibers  of  the  peridental  membrane. 
The  Lamellae  of  the  Cementum  and  Their  Arrangement. — The 

lamellae  of  the  cementum  resemble  those  of  bone,  but  they 
are  very  much  more  irregular  both  in  thickness  and  appear- 
ance. They  may  be  extremely  thin  and  almost  transparent, 
or  they  may  be  quite  thick  and  coarsely  granular.  They 
are  not  nearly  as  easily  observed  as  those  of  bone,  for  in  bone 
the  lamellae  are  marked  off  by  the  lacunae  which  lie  between 
them,  while  in  cementum  the  lacunae  may  be  entirely  absent, 
and  when  present  are  irregularly  placed. 

In  the  gingival  portion  of  the  root  the  lamellae  are  always 
thin  and  very  transparent,  and  lacunae  are  seldom  seen.  The 
entire  thickness  of  the  tissue  is  transparent,  and  the  appear- 
ance of  the  lamellae  can  be  seen  only  by  using  a  very  small 
diaphragm  or  oblique  illumination.  In  this  position  the 
tissue  is  largely  made  up  of  embedded  connective-tissue 
fibers,  which  are,  however,  so  perfectly  calcified  that  they 
cannot  be  demonstrated  in  ground  sections.  In  decalci- 
fied sections  they  are  very  easily  seen. 

The  cementum  becomes  gradually  thicker  in  the  middle 
third  of  the  root,  and  is  thickest  in  the  apical  third.  It  will 
be  seen  that  this  increase  in  thickness  is  caused  chiefly  by 
the  greater  thickness  of  each  individual  lamella.  In  longi- 
tudinal sections  the  cementum  is  often  found  becoming 
suddenly  thicker  at  a  certain  point,  and  if  examined  closely, 
it  will  be  seen  that  each  layer  is  continued  apically,  but  with 
greater  thickness.  Fig.  149  illustrates  this  condition  near  the 
apex  of  the  root.  From  a  study  of  the  lamellae,  therefore,  it 
is  apparent  that  the  entire  root  is  clothed  with  successive 
layers,  and  that  these  layers  are  formed  intermittently,  but 
continue  to  be  formed  as  long  as  the  tooth  is  in  position. 


THE  LAMELLA  OF   THE  CEMENTUM 


191 


In  a  general  way  the  number  of  layers  is  an  index  to  the  age 
of  the  person  at  the  time  the  tooth  was  extracted  (Figs. 
150  and  151).     The  rate  of  formation  is  not  uniform;  for 


Fig.    147 


''■:■''>  .^ 


^^ 


/ 


kv\iiivv\\\\\vM\  \\\\n\Vu\\\\\VV\\wa\ 


^^UiV 


Hjpertrophy  of  the  cementum  on  the  side  of  the  root  of  a  lower  molar  near  the 
neck  of  the  tooth.  From  a  lengthwise  section,  man:  a,  dentine;  h,  cementum;  c, 
fibers  of  peridental  membrane.  From  6  to  c  the  cementum  is  normal  and  the  incre- 
mental lines  fairly  regular,  but  at  d  one  of  the  lamellae  is  greatly  thickened.  At  e 
this  lamella  is  seen  to  be  about  equal  in  thickness  with  the  others.  The  next  two 
lamellae  are  thin  over  the  greatest  prominence,  but  one  is  much  thickened  at  g,  and 
both  at  h.  These  latter  seem  to  partially  fill  the  valleys  which  were  occasioned  by 
the  first  irregular  growth.      (1  in.  obj.) 


Fig.  148 


WMfMMI&^^^MMiM^ 


Hypertrophy  from  root  of  cuspid,  man,  in  which  the  irregularity  is  confined  to 
the  first  lamella:  o,  dentine:  6,  thickened  first  lamella:  c,  subsequent  lamellae,  which 
are  seen  to  be  fairly  regular.      (1  in.  obj.) 


192 


THE  CEMENTUM 


instance,  a  number  of  layers  may  be  formed  within  a  short 
time,  and  again,  a  considerable  time  may  elapse  between  the 
formation  of  one  layer  and  the  next.  The  time,  however, 
does  not  seem  to  determine  the  thickness  of  the  layer. 


Fig.   149 


Apex  of  root  of  an  upper  bicuspid  tooth  with  irregularly  developed  cementum: 
a,  a,  dentine;  b,  b,  pulp  canals.  The  lamellse  of  cementum  are  marked  1,  2,  3,  4,  5, 
6,  7,  8,  9;  d,  d,  d,  absorption  areas  that  have  been  refilled  with  cementum.  It  will 
be  seen  that  the  apices  of  the  roots  were  originally  separate,  but  became  fused  with 
the  deposit  of  the  second  lamella  of  cementum,  and  that  in  this  the  irregular  growth 
began  and  was  most  pronounced.  It  has  continued  through  the  subsequent 
lamellse,  but  in  less  degree.  It  will  also  be  noticed  that  the  absorption  areas,  d,  d,  d, 
have  proceeded  from  certain  lamellse.  That  between  the  roots  has  broken  through 
the  first  lamella  and  penetrated  the  dentine,  and  has  been  filled  with  the  deposit 
of  a  second  lamella.  Other  of  the  absorptions  have  proceeded  from  lamellse 
which  can  be  readily  made  out.  The  small  points,  e,  seem  to  have  been  filled  with 
the  deposit  of  the  last  layer  of  the  cementum,  while  others  have  one,  two,  or  more 
layers  covering  them.      (2  in.  obj.) 


If  a  considerable  number  of  teeth  of  persons  of  twenty 
years  of  age  were  sectioned,  the  lamellse  counted,  and  this 
number  compared  with  the  number  found  in  teeth  extracted 


THE  LAMELLA  OF  THE  CEMEXTUM 


193 


from  persons  of  forty,  a  fairly  regular  increase  in  the  number 
of  layers  will  be  noticed,  and  so  on,  for  fifty,  sixty,  seventy, 
or  eighty  years. 

It  is  important  to  remember  in  connection  with  this 
formation  of  cementum  that  the  teeth  move,  more  or  less, 
under  the  influence  of  natural  forces  throughout  life,  and 
that  every  slight  change  in  position  must  be  accomplished 

Fig.    150 


A  transverse  section  of  a  root  extracted  from  a  young  person.      The  cementum  is 
thin,  but  is  thicker  in  the  grooves  on  the  proximal  sides. 


by  the  formation  of  a  new  layer  of  cementum,  to  reattach 
connective-tissue  fibers  in  new  positions  or  adjust  them  to 
new  directions  of  strain. 

The  first  layer  of  cementum  is  formed  while  the  tooth  is 

still  in  its  crypt,  but  apparently  no  connective-tissue  fibers 

are  calcified  into  it.     This  forms  the  first  apparently  clear 

and   structureless   layer   which   lies   next   to   the   granular 

13 


194 


THE  CEMENTUM 


layer  of  Tomes  (Fig.  138).  Even  in  the  teeth  the  entire 
length  of  whose  roots  are  formed  before  they  begin  to  erupt, 
there  is  no  attachment  until  some  stress  comes  upon  the 
crown.  The  tooth  is  lying  loose  in  its  crypt  and  can  be 
picked  out  with  very  little  force.  Bicuspids  are  often  acci- 
dentally extracted  in  the  extraction  of  temporary  molars. 
x\s  soon  as  the  tooth  comes  through  the  gum  a  new  layer 


Fig.   151 


A  transverse  section  of   a  root  from  an  old  person.      This  root  had  carried 
for  many  years.      The  section  was  cracked  and  one  edge  broken. 


of  cementum  is  formed  over  the  entire  root,  attaching  the 
fibers  to  its  surface,  and  as  the  tooth  moves  occlusally,  layer 
after  layer  is  formed.  This  will  be  considered  again  in 
connection  wdth  the  peridental  membrane. 

The  Lacunse  and  Canaliculi. — The  lacunse  of  the  cementum 
correspond  with  the  lacunse  of  bone.  They  difter  from  those 
of  bone,  however,  in  that  they  are  more  irregular  in  shape, 
size,  position,  and  relation  to  the  lamellae,  and  in  the  number 


THE  CEMENT  CORPUSCLES  195 

and  direction  of  the  canaliculi  radiating  from  them.  In 
bone  the  lacunae  are  fairly  regular  in  shape,  the  long  diameter 
exceeding  the  short  diameter  by  about  one-third.  Sections 
cut  through  their  long  axis  give  an  oval  outline,  the  length  of 
which  is  about  three  times  as  great  as  the  width.  Sections 
cut  through  their  short  axis  give  an  oval  outline,  the  long 
diameter  being  about  twice  that  of  the  short.  The  spaces 
are,  therefore,  flattened  between  the  lamellae.  In  cementum 
there  is  no  regularity  whatever,  either  in  size  or  in  shape. 
Some  are  a  little  larger  than  the  lacunse  in  bone,  some  are 
very  much  smaller.  They  may  be  almost  exactl}^  the  shape 
of  typical  bone  lacunae  or  they  may  be  distorted  into  almost 
any  form,  sometimes  being  almost  stellate,  often  pear- 
shaped,  sometimes  round,  and  occasionally  pyramidal.  The 
lacunae  of  bone  are  fairly  uniformly  placed,  and  lie  between 
one  lamella  and  the  next.^  There  is  no  regularity  in  the 
relation  of  the  lacunae  of  the  cementum  to  the  lamellae. 
They  sometimes  lie  between  one  lamella  and  the  next,  but 
they  are  more  often  entirely  in  the  substance  of  one.  They 
occur  only  where  the  lamellae  are  thick,  and  there  may  be 
large  areas  with  considerable  aggregate  thickness  of  cementum 
in  which  there  are  no  lacunae  at  all. 

The  number  and  direction  of  the  canaliculi  which  radiate 
from  the  lacunae  of  cementum  is  extremely  irregular,  but  in 
general  there  are  more  extending  from  the  lacunae  toward 
the  surface  than  toward  the  dentine. 

The  Cement  Corpuscles. — The  cement  corpuscles  corre- 
spond exactly  to  bone  corpuscles.  They  are  the  cells  found 
in  the  lacunae.  These  are  simply  embedded  cementoblasts 
and  are  typical  connective-tissue  cells.  They  are  made 
up  of  granular  protoplasm  and  contain  a  faintly  staining 
nucleus.  Extensions  of  the  protoplasm  undoubtedly  extend 
into  the  canaliculi.  These  cells  bear  the  same  relation  to 
the  matrix  of  the  cementum  that  bone  corpuscles  do  to  that 

'  This  is  not  absolutely  correct,  there  being  much  more  irregularity  in  the  arrange- 
ment of  the  lacunae  in  thick  subperiosteal  bone  than  in  either  cancellous  or  Haversian 
system  bone.  To  be  strictly  accurate,  the  above  statement  must  be  limited  to  Haver- 
sian system  bone  (Plate  X). 


196  THE  CEMENTUM 

of  bone.  What  this  is  is  not  known  in  any  definite  way,  but 
it  is  known  that  when  bone  corpuscles  are  killed  or  die,  the 
matrix  becomes  a  foreign  body,  and  is  either  absorbed  or 
cut  off  from  the  portion  in  which  the  corpuscles  are  living, 
to  be  absorbed  or  cast  out  as  a  sequestrum.  The  same 
conditions  are  true  of  cementum.  P'or  instance,  there  are 
many  cement  corpuscles  in  the  lacuna  in  the  region  of  the 
apex  of  the  root.  If  this  portion  be  bathed  in  pus  for  a  long 
time,  the  cement  corpuscles  are  killed,  and  the  tissue  becomes 
saturated  with  poisonous  materials,  so  that  tissue  cells 
cannot  lie  in  contact  with  it  and  live.  In  order  to  restore  a 
healthy  condition,  the  necrosed  cementum  must  be  removed 
mechanically  until  tissue  is  reached  with  which  cells  may 
lie  in  physiological  contact  without  injury.  Conditions 
which  can  only  be  understood  through  a  knowledge  of  the 
structure  of  the  tissue  often  arise  in  connection  with  the 
treatment  of  alveolar  abscess.  It  should  always  be  remem- 
bered that  the  treatment  of  an  abscess  is  a  biological  problem. 
The  Embedded  Fibers  of  the  Peridental  Membrane. — The 
embedded  fibers  of  the  peridental  membrane  are  in  the 
strictest  sense  comparable  with  the  fibers  of  Sharpe  in  bone. 
They  are,  however,  in  many  places  much  more  perfectly 
calcified.  To  appreciate  the  relation  of  the  embedded  fibers 
to  the  matrix,  the  tissue  must  be  studied  both  in  ground 
and  decalcified  sections.  For  instance,  in  the  gingival 
portion,  from  the  study  of  ground  sections,  the  presence 
of  embedded  fibers  would  never  be  suspected,  but  if  decal- 
cified sections  are  studied  it  will  be  found  to  be  almost 
entirely  composed  of  calcified  fibers.  In  the  middle  and 
apical  thirds  of  the  root,  w^here  the  lamellse  are  thicker,  the 
calcification  of  these  fibers  is  often  not  as  perfect  as  that 
of  the  rest  of  the  matrix.  In  the  preparation  of  ground  sec- 
tions, therefore,  the  imperfectly  calcified  fibers  shrink  and 
consequently  appear  as  canals  in  the  cementum.  In  fact, 
they  have  often  been  mistaken  for  canals.  They  are  usually 
not  seen  unless  the  section  happens  to  cut  in  their  direction. 
These  will  be  seen  in  many  of  the  illustrations  of  cementum. 
In  Fig.  152  several  layers  are  seen  next  to  the  dentine,  in 


EMBEDDED  FIBERS  OF  PERIDENTAL  MEMBRANE     197 

which  no  fibers  appear,  then  in  several  layers  the  fibers  are 
plainly  seen,  and  finally,  the  surface  layers  show  no  fibers. 
This  probably  means  that  before  and  after  these  layers  were 
formed  there  was  a  change  in  the  position  of  the  tooth  and 
the  fibers  were  all  cut  off  in  this  area  and  reattached  in  a  new 

Fig.    152 


Cementum   near   the   apex   of   the   root:      Gt,  granular  layer  of   Tomes;    L,  lacunae; 
b,  point  at  which  fibers  were  cut  off  and  reattached.      (About  54  X) 


direction,  adapting  them  to  the  new  directions  of  strain. 
It  is  often  necessary  to  study  ground  sections  very  closely 
to  determine  whether  certain  appearances  are  embedded 
fibers  or  canaliculi  radiating  from  the  lacunae.  The  appear- 
ance of  these  fibers  should  be  studied  in  Fig.  153.  It  should 
be  noted  that  wherever  special  stress  is  exerted  upon  a 


198 


THE  CEMENTUM 


bundle  of  fibers  the  cementum  is  thick  around  them.    This 
may  be  seen  in  decalcified  sections  in  Figs.  220, 248  and  Plate 


Fig.    153 


Two  fields  of   cementum   showing  penetrating  fibers:    Gt.  granular   layer  of   Tomes; 
C,  cementum  not  showing  fibers;   F,  penetrating  fibers.      (About  54  X) 


XIV  and  in  ground  sections  in  Figs.  152  and  153.  When  the 
next  layer  is  formed,  if  the  fibers  are  cut  off,  the  additional 
thickness  of  the  last  layer  is  removed.    The  unequal  thick- 


EMBEDDED  FIBERS  OF  PERIDENTAL  MEMBRANE     199 

ness  of  the  last  formed  layer  is  not  seen  in  the  layers  beneath 
it  to  as  great  an  extent. 

Fig.    154 


Record  in  the  calcified  tissue  of  an  absorption  repaired:   D,  dentine;  Cm,  cementum 
filling  absorption  cavity.      (About  40  X) 

Fig.    155 


Thick  lamellae  of   cementum  with  many  lacunae,  filling  an  absorption  in  dentine:   L, 
lacunae;   H,  Howship's  lacunae  filled;   D,  dentine.      (About  250  X) 


200  THE  CEMENTUM 

Absorption  and  Repair  of  the  Cementum. — From  what  has 
ah-eady  been  said  about  the  cementum,  it  will  be  understood 
that  this  tissue  is  continually  undergoing  changes,  that  new 
layers  are  being  added,  and  that  often  before  an  addition  is 
made  there  is  absorption  enough  of  it  at  least  to  cut  off  the 
fibers.  When  an  absorption  occurs  on  the  side  of  a  root 
which  cuts  into  the  dentine,  the  excavation  in  the  dentine  may 
be  filled  by  the  dentine  subsequently  formed  (Figs.  154  and 
155).  From  a  study  of  ground  sections  in  class  work  such 
absorptions  are  not  uncommon.  They  probably  occur  when 
the  cusps  first  come  into  occlusion  in  eruption. 


CHAPTER  XV 

DENTAL  PULP 

Definition. — The  dental  pulp  may  be  defined  as  the  connec- 
tive tissue  occupying  the  central  cavity  of  the  dentine. 

It  is  composed  of  embr^^onal  connective  tissue  which  is 
more  closely  related  to  the  tissue  occupying  the  spaces  of 
cancellous  bone  than  to  any  other. 

Functions. — The  functions  of  the  dental  pulp  are: 

1.  A  vital  function,  the  formation  of  dentine. 

2.  A  sensory  function  responding  to  thermal  change  and 
chemical  and  traumatic  irritation. 

J^ital  Function. — The  vital  function  is  the  formation  of 
dentine  and  is  performed  by  the  layer  of  odontoblasts. 
These  cells  also,  by  means  of  their  dentinal  fibrils,  maintain 
the  same  relation  to  the  dentine  matrix  that  the  bone  and 
cement  corpuscles  bear  to  the  matrix  of  bone  and  cementum. 
When  the  pulp  is  removed  from  a  tooth  its  dentine  becomes 
dead  dentine  in  the  same  sense  that  bone  in  which  the  bone 
corpuscles  have  been  killed  is  necrosed  bone.  That  there 
is  a  constant  reaction  between  the  protoplasm  of  the  odonto- 
blasts and  the  substance  of  the  dentine  matrix,  or  that  the 
presence  of  the  living  protoplasm  is  necessary  to  prevent 
degeneration  of  the  matrix,  is  evidenced  by  the  changes  in 
the  physical  properties  of  the  dentine  after  the  pulp  has 
been  lost.  That  the  tooth  remains  functional  after  the  loss 
of  the  pulp  is  due  to  the  fact  that,  except  at  the  minute 
foramina,  the  dentine  is  not  in  physiologic  contact  with 
any  tissue  excepting  enamel  and  cementum,  and  that  the 
cementum  attaches  the  tooth  to  the  surrounding  tissues, 
receiving  its  nourishment  from  the  surface  and  not  from  the 
dentine. 


202  DENTAL  PULP 

When  the  pulp  is  removed  and  its  place  filled  by  a  non- 
irritating  material,  the  dentine  becomes  entirely  encased 
in  cementum,  the  foramina  probably  being  covered  over  as 
the  subsequent  lamellae  are  formed.  The  author  wishes  to 
emphasize,  however,  the  vital  relations  of  the  pulp  to  the 
dentine  matrix.  Dead  dentine  is  never  as  good  as  living 
dentine,  consequently  a  tooth  from  which  the  pulp  has  been 
removed  can  never  be  considered  just  as  good  as  one  with 
the  living  pulp. 

The  production  of  the  dentine  matrix  is,  of  course,  the 
principal  part  of  the  vital  function  of  the  pulp.  It  is  begun 
in  the  development  of  the  tooth  before  the  dental  papilla 
is  converted  into  the  dental  pulp,  by  being  enclosed  in  the 
dentine  formed.  After  the  tooth  is  fully  formed  the  pulp 
retains  its  ability  to  build  dentine  matrix  as  long  as  it 
retains  vitality,  but  this  function  is  exercised  only  in  response 
to  conditions  of  environment  which  are  probably  excited 
through  the  intervention  of  its  sensory  function  responding 
to  thermal  change  and  chemical  irritation.  The  sensory 
function  causes  a  trophic  impulse  which  is  manifested  by  the 
production  of  another  portion  of  dentine  matrix  reducing  the 
size  of  the  pulp  chamber.  That  this  is  a  reflex  and  not  purely 
a  local  matter  is  indicated  by  the  fact  that  formations  of 
dentine  occur  in  one  tooth  when  the  irritation  is  in  another, 
and  apparently  the  irritation  of  one  tooth  will  excite  dentine 
formation  in  all  of  the  teeth  on  that  side,  at  least  in  some 
instances.  On  the  other  hand,  purely  local  responses  are 
found  where  a  few  odontoblasts  respond  to  the  irritation 
of  their  fibrils  by  the  formation  of  dentine. 

This  matter  has  been  referred  to  under  the  heading  of 
secondary  dentine,  and  it  is  best  studied  by  the  record  it 
leaves  in  the  formed  tissue. 

The  Sensory  Function. — In  regard  to  sensation,  the  pulp 
resembles  an  internal  organ,  as  in  its  normal  condition  it  is 
always  enclosed  in  the  cavity  of  the  dentine.  It  has  no  sense 
of  touch  or  localization,  and  responds  to  stimuli  only  by 
sensations  of  pain.  The  pain  is  usually  located  correctly 
with  reference  to  the  median  line,  but  apart  from  that  it  is 


STRUCTURAL  ELEMENTS  203 

located  only  as  it  is  referred  to  some  known  lesion.  If 
several  pulps  were  exposed  on  the  same  side  of  the  mouth, 
and  in  teeth  of  both  the  upper  and  lower  arches,  so  that  they 
could  be  irritated  without  impressions  reaching  the  peri- 
dental membrane,  if  the  patient  were  blindfolded  it  would 
be  impossible  for  him  to  tell  which  of  the  pulps  was  touched. 
This  characteristic  becomes  extremely  important  in  diag- 
nosis. 

The  pain  originating  from  a  tooth  pulp  may  be  referred 
to  the  wrong  tooth  or  to  almost  any  point  on  the  same  side 
supplied  by  the  fifth  cranial  nerve. 

The  dental  pulp  is  especially  sensitive  to  changes  in 
temperature,  amounting  almost  to  a  temperature  sense, 
having  no  exact  parallel  in  any  other  tissue  of  the  body. 
This  does  not  amount  to  a  recognition  of  heat  or  cold  as 
such,  but  a  special  resentment  to  sudden  changes.  For 
instance,  if  a  tooth  is  isolated  and  so  protected  by  non- 
conductors that  the  soft  tissues  cannot  be  stimulated,  and  a 
jet  of  hot  and  then  cold  water  be  thrown  upon  its  crown, 
it  will  respond  to  each  with  a  sharp  sensation  of  pain,  but 
the  patient  cannot  tell  which  is  hot  and  which  is  cold.  It 
is  the  sudden  change  that  produces  the  reaction.  This  is 
the  basis  of  very  important  differential  diagnoses,  for,  us  is 
true  with  most  organs,  in  pathologic  conditions  its  sensory 
function  is  exaggerated. 

Histogenesis. — The  dental  pulp  is  the  remains  of  the  dental 
papilla.  The  dental  papillae  for  the  temporary  teeth  appear- 
ing in  the  mesodermal  tissue  of  the  jaw  arches  very  early  in 
fetal  life.  The  cellular  elements  are  at  first  very  closely 
placed  and  large,  but  they  grow  smaller  and  take  on  the 
typical  form  of  pulp  cells  as  the  intercellular  substance  is 
increased.  By  the  sixteenth  week  the  dental  papillae  for 
the  temporary  teeth  are  covered  by  a  layer  of  tall  columnar 
cells,  which  will  begin  the  formation  of  dentine  about  that 
time.  After  the  beginning  of  dentine  formation  the  trans- 
ition from  the  dental  papillae  to  the  dental  pulp  is  very 
gradual,  and  it  would  be  impossible  to  draw  any  sharp  line 
of  demarcation  between  them. 


204 


DENTAL  PULP 


Structural  Elements. — The  structural  elements  of  the  den- 
tal pulp  are: 

1.  Odontoblasts. 

2.  Connective-tissue  cells. 

3.  Intercellular  substance. 

4.  Bloodvessels. 

5.  Nerves. 

The  Odontoblasts. — The  odontoblasts  are  tall  columnar 
cells  which  form  the  outer  layer  of  the  pulp  adjacent  to  the 
dentine,  and  from  which  cytoplasmic  fibrils  extend  into  the 
dentinal  tubules. 

Fig.   156 


Odontoblasts   and   forming   dentine:     E,   forming   enamel;    D,   forming   dentine;   O, 
odontoblasts;   Dp,  body  of  dental  papilla.      (From  photomicrograph  by  Rose.) 


The  character  of  the  odontoblasts  changes  very  greatly 
with  the  age  of  the  tissue,  and  the  activity  of  dentine  forma- 
tion. While  the  primary  dentine  is  being  formed  they  are 
tall  columnar  cells,  each  containing  a  large  oval  nucleus, 


THE  ODONTOBLASTS  205 

rich  in  chromatin  and  located  in  the  pulpal  third  of  the 
cell.  From  the  dentinal  end  of  the  cell  cytoplasm  is  con- 
tinued, without  any  line  of  demarcation,  into  the  dentinal 
tubule  as  the  dentinal  fibril.  In  some  instances  two  fibrils 
may  be  sent  from  a  single  odontoblast.  The  character  of 
the  odontoblast  is  beautifully  seen  in  Fig.  156,  a  photograph 
by  Professor  Rose. 

Fig.  157 


r  t*:*  T 


%  „«l 


^ts>  g^   j^'i^       ..^^M 


Odontoblasts.  The  section  cuts  obliquely  through  the  odontoblasts:  F,  fibrils; 
A^,  nuclei  of  odontoblasts;  A'',  nuclei  of  connective-tissue  cells;  W,  lajer  of  Weil, 
not  well  shown.      (About  SO  X) 

After  the  tooth  is  erupted,  but  while  the  formation  of 
dentine  is  actively  going  on,  the  odontoblasts,  while  some- 
what smaller,  retain  the  same  typical  appearance.  They  may 
be  easily  demonstrated  either  in  decalcified  sections  or  by 
removing  pulps  from  the  pulp  chamber  of  freshly  extracted 
teeth.  Professor  Salter  has  described  two  sets  of  processes 
besides  (Fig.  157)  the  dentinal  fibril  process.  As  a  result  of 
teasing  the  fresh  pulps,  he  considered  that  fine  projections  of 


W 


206 


DENTAL  PULP 


Fig.   158 


Diagram  of  odonto- 
blasts and  dentinal 
fibrils.     (C.H.Stowell.) 


the  protoplasm  extended  from  the  sides 
of  the  cells,  uniting  them  to  the  adjoining 
odontoblasts  (Fig.  158).  These  he  called 
the  lateral  processes.  He  also  described 
cytoplasmic  projections  from  the  pulpal 
end  of  the  odontoblasts  into  the  laj'er  of 
Weil.  It  is  probable  that  these  appear- 
ances were  the  result  of  teasing,  and  are 
not  true  structural  characteristics,  as  the 
work  of  other  investigators  has  not  con- 
firmed their  presence.  It  is  easy  to  under- 
stand how  teasing  the  cells  apart  might 
produce  appearances  which  might  be  inter- 
preted as  processes,  but  careful  work  upon 
sections  does  not  show  their  presence. 

In  old  pulps  where  the  formation  of 
dentine  has  been  intermittent  and  very 
infrequent  for  a  long  time,  the  odonto- 
blasts are  smaller,  lose  their  columnar  form 
more  or  less,  and  become  pear-shaped  or 
globular. 

As  dentine  is  one  of  the  most  highly 
specialized  connective  tissues,  the  odonto- 
blasts are  among  the  most  highly  differen- 
tiated connective-tissue  cells.  They  are 
the  only  connective-tissue  cells  of  colum- 
nar form.  Morphologically,  they  are  very 
similar  to  columnar  epithelium,  but  epi- 
thelial cells  never  have  such  processes  as 
the  dentinal  fibrils.  Occasionally,  in  young 
and  actively  growing  bone,  osteoblasts  are 
found  which  are  distinctly  columnar  in 
form,  but  they  are  never  as  tall  as  the 
odontoblasts,  and  the  nucleus  is  more 
nearly  in  the  centre  of  the  cell.  In  the 
case  of  the  osteoblast  the  cytoplasmic 
processes  which  extend  into  the  canaliculi 
correspond  to  the  dentinal  fibril  process 
of  the  odontoblast.  '  The  homologies  be- 
tween  the   osteoblasts  and  the   odonto- 


THE  MEMBRANA  EBORIS  207 

blasts  have  often  been  lost  sight  of  in  the  discussions  over 
the  character  of  the  latter  and  their  relation  to  the  forma- 
tion and  sensitiveness  of  the  dentine. 

The  Membrana  Eboris. — The  odontoblasts  form  a  single 
layer  of  cells  on  the  surface  of  the  pulp  in  contact  with  the 
dentine.  This  layer  was  very  early  recognized  to  be  related 
to  the  formation  of  the  dentine,  and  was  called  the  memhrana 
eboris,  or  the  membrane  of  the  ivory.  The  name  has  no 
importance  now  except  as  it  is  found  in  the  literature. 

Size  of  the  Odontoblasts. — From  what  has  been  said  it  will 
be  recognized  that  the  size  and  shape  of  the  odontoblasts 
vary  greatly  in  different  sections.  This  is  true  not  only  of 
pulps  from  different  animals,  and  pulps  at  different  periods 
of  development,  but  of  different  parts  of  the  same  pulp.  In 
the  coronal  portion  of  a  pulp  from  a  fully  developed  tooth, 
but  one  in  which  the  formation  of  dentine  is  still  going  on, 
the  average  measurements  would  be  about  5/^  in  diameter 
and  25  to  30/^-  in  height.  During  early  stages  of  dentine 
formation,  before  the  crown  is  fully  formed,  they  are  con- 
siderably larger  and  taller,  and  in  the  pulps  of  a  calf  they  are 
much  larger  than  in  smaller  animals  and  man.  In  a  con- 
stricted pulp,  as,  for  instance,  in  the  mesial  root  of  a  lower 
first  molar,  the  odontoblasts  on  the  constricted  sides  will 
be  shorter  and  relatively  thicker  than  on  the  buccal  and 
lingual,  where  the  long  axis  of  the  cell  is  in  the  direction  of 
the  long  diameter  of  the  pulp,  but  this  simply  means  that 
the  formation  of  dentine  on  the  constricted  side  is  relatively 
farther  advanced  than  on  the  buccal  and  lingual,  and  the 
cells  show  older  phases.  It  is  evident  that  the  supply  of 
nourishment  to  the  cells  in  the  constricted  portions  is  more 
imperfect,  and  that  the  ones  farthest  from  the  main  vessels 
are  most  affected,  so  that  dentine  formation  is  slowed  and 
made  more  imperfect  here,  while  it  still  continues  in  full  vigor 
around  the  expanded  portions  of  the  pulp.  This  has  been 
spoken  of  in  connection  with  the  study  of  the  dentine  (see 
Figs.  145  and  146). 

Origin  of  the  Odontoblasts. — The  odontoblasts  are  special- 
ized connective-tissue  cells.    It  is  therefore  to  be  expected 


208  DENTAL  PULP 

that  they  should  be  formed  from  undifferentiated  connective- 
tissue  cells  as  osteoblasts  are  formed  from  similar  cells  of  the 
inner  laj^er  of  the  periosteum.  The  odontoblasts  are  there- 
fore developed  from  embryonal  cells  deeper  in  the  pulp  which 
take  their  place  in  the  odontoblastic  layer.  This  probably 
explains  the  appearance  of  some  sections,  and  also,  the  author 
believes,  the  views  of  some  men  in  regard  to  the  odontoblasts 
and  the  dentinal  fibrils.  In  some  sections  from  old  pulps  the 
odontoblasts  seem  to  be  in  an  incomplete  layer,  and  their 
form  is  more  like  that  of  typical  connective-tissue  cells. 

Connective-tissue  Cells. — The  cells  in  the  dental  pulp, 
aside  from  the  odontoblasts,  are  typical  connective-tissue 
cells  such  as  are  found  in  embryonal  tissue.  They  are  of 
three  forms — round,  spindle-shaped,  and  stellate.  In  the 
crown  or  bulbous  portion  the  cells  are  mostly  stellate,  while 
in  the  root  portion  they  are  largely  spindle-shaped,  with  the 
axis  of  the  spindle  parallel  with  the  canal.  It  seems  difficult 
for  students  to  get  an  idea  of  their  arrangement,  and  the 
nucleus  is  often  mistaken  for  the  entire  cell.  The  cells  do  not 
lie  in  contact  in  a  compact  tissue,  but  are  widely  scattered 
in  the  intercellular  substance.  There  is  a  small  ovoid 
nucleus,  which  takes  the  stain  deeply,  surrounded  by  a  mass 
of  granular  protoplasm  stretching  away  into  very  fine  threads. 
In  the  spindle-shaped  cells  the  protoplasm  is  stretched  out 
in  only  two  directions.  In  the  stellate  cells  there  may  be 
three,  four,  or  more,  stretching  away  in  any  direction.  Plate 
VIII  was  very  carefully  drawn  with  the  camera  lucida  so  as 
to  represent  accurately  the  number,  size,  and  position  of  the 
cells  in  that  field  as  seen  with  the  yV  oil  immersion.  It  is 
very  difficult  in  a  drawing  to  represent  the  third  dimension  of 
space,  and  to  show  that  some  of  the  processes  are  extending 
in  a  plane  at  right  angles  to  the  paper.  An  idea  of  this  can 
only  be  obtained  by  the  very  careful  use  of  the  fine  adjust- 
ment while  studying  the  cells  with  the  high  power. 

The  round  cells  are  probably  white  blood  corpuscles  or 
undifferentiated  connective-tissue  cells  which  may  develop 
either  into  stellate  or  spindle-shaped. 


PLATE   VII 


.   ft* 


^  * 


v^ 


A  Field  from  the  Coronal  Portion  of  the  Pulp  froni 
a  Hunian  Molar. 

In  the  corner  the  stage  micrometer  shows  y^o  of  ^  niilHmeter  drawn 
vvith  the  same^lens.  The  field  sho^A^s  the  branching  of  a  bloodvessel  and 
the  connective-tissue  cells  of  the  pialp.  Drawn  from.  J^  oil-ininiersion 
lens  with  camera  lucida.      (About  12,000  X) 


THE  BLOODVESSELS  209 

The  Arrangement  of  the  Cells. — Immediately  beneath  the 
layer  of  odontoblasts,  for  a  space  about  one-half  or  two-thirds 
as  wide  as  the  odontoblastic  layer,  the  cells  are  very  scarce, 
making  a  clear  line  in  many  sections.  This  is  known  as  the 
layer  of  Weil,  and  contains  many  fine  nerve  fibers  which 
are  not  stained  by  ordinary  methods.  Beyond  the  layer  of 
Weil  for  a  space  perhaps  twice  as  wide  as  the  height  of  the 
odontoblasts,  the  cells  are  very  closely  placed.  Through  the 
remainder  of  the  pulp  they  are  much  more  widely  but 
comparatively  evenly  scattered. 

The  Intercellular  Substance. — Very  little  is  really  known 
about  the  character  of  the  intercellular  substance  of  the 
pulp.  It  contains  few  fibers,  and  these  in  no  way  resemble 
bundles  of  white  or  elastic  connective  tissue.  The  appear- 
ance in  the  section  is  more  as  if  a  structureless  gelatinous 
material  had  been  coagulated  by  the  reagents. 

There  are,  of  course,  connective-tissue  fibers  in  connection 
with  the  walls  of  the  larger  bloodvessels  and  nerves,  and  to  a 
certain  extent  in  the  gelatinous  material.  In  studying  the 
intercellular  substance  in  the  sections  it  is  necessary  to 
remember  that  it  is  filled  with  the  protoplasmic  projections 
from  the  cells,  and  these  are  stained,  appearing  like  fibers 
in  the  matrix.  There  is  need  for  further  investigation  of 
the  character  of  the  intercellular  substance. 

The  Bloodvessles. — The  dental  pulp  is  an  extremely  vas- 
cular tissue,  and  the  arrangement  of  the  vessels,  the  structure 
of  their  walls,  and  the  nature  of  the  intercellular  substance 
through  which  they  run  render  the  tissue  especially  sus- 
ceptible to  the  pathological  conditions  which  are  associated 
with  alterations  in  the  circulation. 

Usually  several  arterial  vessels  enter  the  pulp  through 
foramina  in  the  region  of  the  apex.  These  vessels  have  their 
origin  in  the  rich  vascular  network  of  the  cancellous  bone 
(Chapter  on  Peridental  Membrane).  The  arteries  follow 
the  central  portion  of  the  pulp,  giving  off  many  branches 
as  they  pass  occlusally,  and  finally  form  a  very  rich  plexus 
of  capillaries  near  the  surface  of  the  pulp.  From  these  capil- 
laries the  blood  is  collected  into  the  veins,  which  follow 
14 


210 


DENTAL  PULP 


Fig.   159 


*h 


I 


A  section  through  the  apex  of   a  root  showing  three  foraminse,  A,  B,  and  C. 


THE  BLOODVESSELS 


211 


courses  parallel  to  the  arteries,  leaving  the  pulp  through  the 
same  foramina  in  the  region  of  the  apex.  It  is  important  to 
notice  that  an  artery  is  entering  and  a  vein  leaving  the  tissue 


Fig.  160 


..•  \ 


Diagram  of   the  bloodvessels  of   the  pulp.      (Stowell.) 

through  very  minute  canals  in  the  calcified  dentine  (Fig. 
159).  Dr.  Stowell  has  made  a  very  beautiful  diagram  of 
the  arrangement  of  the  bloodvessels  in  a  single-rooted  tooth, 
which  is  shown  in  Fig.  160.      Preparations  such  as  would 


212  DENTAL  PULP 

reproduce  this  diagram  can  be  made  by  injecting  the  blood- 
vessels with  an  inert  material  and  destroying  the  soft  tissues 
by  artificial  digestion. 

Structure. — The  delicacy  of  the  walls  of  the  bloodvessels 
is  one  of  the  most  striking  histologic  characteristics  of  the 
dental  pulp.  The  largest  arteries  show  only  a  few  muscle 
fibers  in  the  media  and  a  very  slight  condensation  of  fibrous 
tissue  for  an  adventitia.  There  is  no  distinct  boundary 
between  the  capillaries  and  the  veins,  and  the  vessels  con- 
tinue to  have  only  a  wall  of  endothelial  cells  after  they  have 
reached  a  size  much  greater  than  that  of  capillaries.  Because 
of  this  peculiarity  of  structure  the  statement  is  to  be  found 
in  many  text-books  of  histology  that  the  largest  capillaries 
in  the  body  are  found  in  the  dental  pulp.  These  vessels 
should  probably  not  be  considered  as  capillaries,  but  as 
veins  whose  walls  have  the  structure  of  capillaries.  Even 
in  the  largest  veins  the  media  is  very  imperfect,  and  there  is 
only  a  slight  condensation  of  fibrous  tissue  to  represent  the 
adventitia.  This  peculiarity  of  the  bloodvessel  walls  in  the 
pulp  renders  the  tissue  peculiarly  susceptible  to  hyperemia 
and  inflammation. 

Fig.  161  is  a  photograph  of  a  bloodvessel  whose  size  can 
be  estimated  from  the  number  of  red  corpuscles  seen  in  it, 
and  the  wall  is  made  up  of  a  single  layer  of  endothelial 
cells.  There  is  no  indication  of  either  media  or  adventitia. 
The  intercellular  substance  of  the  pulp  being  of  gelatinous, 
semifluid  character,  gives  no  support  to  these  delicate  walls. 

In  Plate  VIII  the  author  has  drawn  very  carefully,  with 
the  camera  lucida,  using  a  yV  immersion  lens,  a  field  showing 
the  branching  of  a  small  bloodvessel.  The  size  of  the  endo- 
thelial cells,  position  of  their  nuclei  in  the  wall  of  the  vessel, 
and  the  size,  position,  and  shape  of  the  connective-tissue  cells, 
are  represented  as  accurately  as  possible.  The  field  is  from 
the  coronal  portion  of  the  pulp  of  a  human  molar.  The  caliber 
of  such  a  vessel  as  this  would  depend  almost  entirely  upon 
the  blood  pressure.  The  endothelial  cells  will  stretch  to  a 
very  considerable  extent  under  increased  pressure,  becoming 
very  thin  at  all  points  except  around  the  nucleus.     When 


THE  BLOODVESSELS 


213 


the  pressure  is  decreased  the  contractiHty  of  the  protoplasm 
pulls  the  cells  together,  making  it  thicker  and  less  in  diameter. 
It  is  very  important  to  remember  these  facts  in  connection 
with  hyperemia  of  the  dental  pulp.     It  is  difficult  in  such 


Fig.  161 


A  pulp  bloodvessel,  showing  the  thin  wall:  C,  blood  corpuscles  in  the  vessel; 
Bl,  bloodvessel  wall  showing  nuclei  of  endothelial  cells;  N,  nuclei  of  connective- 
tissue  cells  in  the  bodj-  of  the  pulp;  I,  intercellular  substance,  showing  a  few 
fibers.      (About  200  X) 


an  illustration  to  give  any  representation  of  the  third  dimen- 
sion of  space,  which  is  essential  to  a  real  understanding  of 
the  connective-tissue  cells  of  the  pulp.  These  are  bits  of 
cytoplasm  with  a  nucleus  forming  a  small  irregular  central 


214  DENTAL  PULP 

mass,  from  which  the  cytoplasm  is  stretched  away  in  all 
directions  through  the  intercellular  substance,  ending  in 
very  fine  threads. 

Plate  IX  is  drawn  in  the  same  way  from  a  transverse  section 
of  the  pulp  of  an  unerupted  tooth  of  a  sheep.  The  vessels 
are  all  cut  transversely  and  are  seen  crowded  with  red  blood 
corpuscles.  They  are  not  distended,  and  some  show  slight 
condensation  of  fibrous  tissue  around  them. 

In  a  normal  pulp  there  are  many  capillaries  so  small  that 
a  single  corpuscle  passes  them  with  difficulty,  but  in  patho- 
logic conditions  they  become  distended  to  many  times 
their  normal  diameter.  All  investigators  have  agreed  in 
finding  no  lymphatic  vessels  in  the  pulp.  This  is  also  an 
important  fact  in  connection  with  pathologic  conditions. 

The  Nerves  of  the  Dental  Pulp. — Few  subjects  in  connection 
with  dental  histology  have  received  more  attention  than 
the  distribution  of  the  nerves  of  the  dental  pulp,  especially 
in  relation  to  the  sensitiveness  of  the  dentine.  Support 
for  almost  any  idea  can  be  found  in  the  literature,  but  many 
of  the  conditions  described  have  been  shown  to  be  errors  in 
microscopic  interpretation,  and  many  others  have  failed  to 
receive  support  by  reinvestigation.  The  most  recent  work 
upon  this  subject  was  done  ten  or  twelve  years  ago  by 
Prof.  Carl  Huber,  of  Ann  Arbor.  The  author  has  repeated 
some  of  his  work,  and  has  never  seen  any  specimen  that 
was  contradictory  to  his  statements.  Usually  three  or  four 
nerve  trunks  enter  the  dental  pulp  through  the  foramina. 
These  contain  from  eight  or  ten  to  thirty  or  forty  medullated 
nerve  fibers.  They  pass  occlusally  through  the  central 
portion  of  the  pulp,  but  almost  immediately  begin  to  give 
off  branches,  which  pass  toward  the  periphery,  branching 
and  anastomosing  in  their  course.  Most  of  the  fibers  lose 
their  medullary  sheath  very  soon  after  leaving  the  nerve 
trunk,  proceeding  as  beaded  fibers,  made  up  of  an  axis 
cylinder  with  nuclei  scattered  along  it.  A  bundle  of  such 
fibers,  breaking  up  to  be  distributed  to  one  horn  of  the 
pulp,  is  shown  in  Fig.  162.  Other  fibers  retain  their  medullary 
sheath,  following  an  independent  course  through  the  pulp 


PLATE    IX 


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A  Field  froni  the  Pulp  of  an  Unerupted  Tooth  of  a  Sheep. 

The  bloodvessels  are  cut  transversely.      (About  lOOO  X) 


THE  NERVES  OF   THE  DENTAL  PULP 


215 


tissue,  until  they  reach  the  layer  of  Weil,  where  the  sheath  is 
lost  and  they  join  the  plexus  of  beaded  fibers  Wmg  in  this  posi- 
tion (Fig.  1G3).  From  the  plexus  in  the  layer  of  Weil  beaded 
fibers  are  given  off,  passing  between  and  around  the  odonto- 
blasts, forming  a  network  around  each  cell,  and  even  passing 
over  on  to  the  end  of  the  cell  between  it  and  the  dentine, 
but  they  have  never  been  followed  into  the  dentinal  tubules. 
In  no  instance  and  by  no  method  that  he  has  employed,  has 
Dr.  Huber  been  able  to  demonstrate  nerve  fibers  in  the 
dentinal  tubules. 


Fig.   162 


x^^^^ 


Nerve  fibers  in  pulp  from  a  human  mol 


(About  500  X) 


The  sensitiveness  of  the  dentine,  in  view  of  these  obser- 
vations, is  due  to  the  presence  of  living  fibrils,  connected 
with  living  odontoblasts  which  are  in  physiologic  connec- 
tion with  nerve  fibers.  It  is  interesting  to  note  that  this  is 
the  only  instance  in  which  a  connective-tissue  cell  is  inter- 
mediate between  the  outside  world  and  the  nerve  fiber.    In 


216 


DENTAL  PULP 


all  other  instances  an  epithelial  cell  is  intermediate  between 
the  environment  and  the  nervous  system.  The  sensitiveness 
of  the  dentine  is  therefore  due  to  the  irritability  of  the  cyto- 
plasm of  the  fibril,  transmitted  through  the  continuity  of 
cytoplasm  to  the  odontoblasts  and  their  reaction  upon  the 
surrounding  nerve  fibers.  The  irritation  to  the  fibril  may 
be  either  traumatic,  chemical,  or  thermal.  For  instance,  salt 
is  sprinkled  on  exposed  living  dentine,  and  a  sharp  sensa- 
tion of  pain  is  the  result.    It  may  be  supposed  that  chemical 


Fig.  163 

A 

h  •■ 

B 

u  ■ 

C 

H 

D 

A  ,|  1  j     \  I:]:-:  I 

Rose's  diagram  of   nerves  and  bloodvessels  of   the  pulp. 


changes  are  set  up  in  the  cytoplasm  of  the  fibril  which  excite 
changes  in  the  cytoplasm  of  the  odontoblasts.  These  react 
upon  the  cytoplasm  of  the  nerve  fiber,  and  so  are  transmitted 
to  the  nerve  centre,  being  recognized,  in  consciousness,  as  a 
sensation  of  pain.  In  the  same  way  traumatic  irritation 
caused,  for  instance,  by  the  cutting  of  dentine  with  a  steel 
instrument  sets  up  changes  in  the  fibril  in  the  same  fashion. 
It  is  impossible  to  conceive  of  any  vital  activity  of  cyto- 
plasm otherwise  than  as  a  form  of  chemical  action  or  molec- 
ular or  atomic  movement  of  its  substance. 


THE  NERVES  OF   THE  DENTAL  PULP  217 

Certain  clinical  facts  are  well  explained  by  these  structural 
facts.  It  is  often  noted  in  the  preparation  of  cavities  that 
the  dentine  is  most  sensitive  at  the  dento-enamel  junction. 
This  would  be  expected  when  it  is  recalled  that  at  the  dento- 
enamel  junction  the  dentinal  tubules  fork  and  the  fibrils 
anastomose,  so  that  an  irritation  to  a  few  fibrils  is  not 
simply  transmitted  to  their  odontoblasts  and  the  nerve 
endings  in  contact  with  them,  but  to  all  the  fibrils,  and  so  to 
the  nerves  in  contact  "^ith  all  of  the  odontoblasts.  The 
presence  of  dilute  acids  render  the  cytoplasm  of  the  fibrils 
much  more  irritable.  The  dentine  in  a  carious  condition 
is,  therefore,  much  more  sensitive  than  that  in  a  sound  or 
normal  area.  The  sensitiveness  of  extremely  hypersensitive 
dentine  can  often  be  greatly  reduced,  if  not  entirely  over- 
come, by  cleansing  the  cavity  thoroughly,  washing  with 
tepid  water  followed  by  a  dilute  alkali,  drying  and  sealing 
for  a  few  days,  when  it  will  be  usually  found  that  excavation 
can  be  carried  out  without  excessive  pain.  The  sealing  must 
be  perfect.  If  it  is  leaky  the  cavity  will  be  more  sensitive 
than  ever  at  the  end  of  the  delay. 

Teeth  in  which  the  size  of  the  pulp  chamber  has  been 
reduced  by  the  formation  of  secondary  dentine  are  usually 
much  less  sensitive.  By  this  formation,  as  has  been  seen 
in  the  chapter  on  dentine,  many  of  the  tubules  are  cut  off 
and  man}'  of  the  fibrils  reach  the  pulp  only  by  anastomosing 
with  a  few  in  the  later  formed  dentine.  The  transmission 
to  the  nerves  of  the  pulp  is  thus  made  more  difiicult  and 
imperfect.  In  all  considerations  of  the  sensitiveness  of 
dentine,  the  purely  subjective  and  hysterical  symptoms 
must  be  carefully  watched  for.  In  many  cases  slight  sensa- 
tions are  so  magnified  by  fear  and  expectation  as  to  be 
considered  intolerable.  In  such  cases  the  diversion  of 
attention  and  the  skilful  use  of  suggestions  are  of  more 
value  when  coupled  "\\'ith  delicacy  of  manipulation  and  opera- 
tive skill  than  any  means  of  obtunding.  In  such  cases, 
although  the  operator  is  positive  that  the  sensations  are 
slight,  it  ^^^ll  never  do  any  good  to  tell  the  patient  so,  or  to 
argue  that  what  is  being  done  cannot  hurt.    They  must  be 


218  DENTAL  PULP 

made  to  believe  fully  that  something  has  been  done  to 
destroy  the  sensitiveness,  and  then  the  attention  must  be 
concentrated  upon  something,  while  the  excavation  is 
lightly  and  skilfully  performed.  It  makes  very  little 
difference  what  is  done,  but  it  must  attract  the  attention  in 
order  to  plant  the  belief  that  the  sensitiveness  has  been 
removed,  and  then  the  attention  must  be  diverted  until 
the  manipulation  is  completed. 

The  nerves  of  the  pulp  not  only  respond  with  sensations 
of  pain  from  the  irritation  of  the  fibrils  in  the  dentinal 
tubules,  but  because  of  their  confinement  in  a  calcified 
chamber  and  the  semifluid  nature  of  the  tissue,  they  are 
very  sensitive  to  pressure,  either  increased  or  decreased. 
The  normal  response  to  changes  of  temperature,  as  well  as 
most  of  the  pain  in  pathologic  conditions  of  the  pulp, 
are  probably  caused  by  changes  of  pressure,  through  dis- 
turbance of  the  blood  circulation  of  the  tissue.  The  nerves 
of  the  pulp  control  the  walls  of  the  arteries  through  the 
vasomotor  reflexes,  and  also  by  trophic  fibers  control  the 
functional  activity  of  the  odontoblasts  in  the  formation  of 
the  dentine. 

In  a  single  tooth  the  irritation  resulting  from  a  carious 
cavity  is  found  to  cause  the  formation  of  dentine  not  simply 
in  the  region  reached  by  the  irritated  fibrils,  but  upon  the 
entire  wall  of  the  pulp  chamber  and  apparently  also  in  other 
teeth.  It  has  seemed  possible  to  the  author  that  in  some 
instances  osmotic  conditions  might  be  a  factor  in  the  pro- 
duction of  pain  in  the  pulp,  especially  in  the  early  stages  of 
caries. 


CHAPTER  XVI 

STRUCTURAL  CHANGES  IX  THE  PATHOLOGY  OF  THE 

PULP 

Because  of  its  structural  peculiarities,  as  well  as  the  fact 
that  it  is  a  tissue  of  embryonal  character  whose  function  has 
been  chiefl}'  performed,  the  dental  pulp  is  specially  sus- 
ceptible to  certain  pathologic  conditions  which  produce 
structural  changes.  These  conditions  are  hyperemia,  inflam- 
mation, suppuration,  and  various  forms  of  tissue  degener- 
ation. The  dental  pulp  ofters  specially  good  opportunities 
for  a  study  of  the  tissue  changes  that  are  characteristic  of 
these  conditions,  because  in  the  normal  state  the  tissue  ele- 
ments are  comparatively  widely  scattered  in  an  almost  struc- 
tureless intercellular  substance.  The  changes  in  the  blood- 
vessels, the  passage  of  cellular  elements  of  the  blood  through 
the  bloodvessel  walls  and  what  becomes  of  them  after  they 
enter  the  tissue  can  therefore  be  followed  more  easily  than 
in  tissues  which  are  crowded  with  cells. 


HYPEREMIA 

Hyperemia  is  defined  as  an  increased  amount  of  blood 
in  a  part.  It  is  usually  divided  into  active  and  passive. 
In  active  hyperemia  the  increase  in  the  amount  of  blood 
is  due  to  the  enlargement  of  the  arteries  supplying  the  part, 
or  the  increase  of  blood  pressure,  or  both.  In  passive 
hyperemia  it  is  due  to  an  obstruction  of  the  veins,  so  that 
the  blood  is  not  allowed  to  escape  as  freely  from  the  part. 
In  case  of  the  dental  pulp  the  conditions  which  cause  an 
active  hyperemia  produce,  at  the  same  time,  a  passive  one. 
H^^eremias  are  also  classified  as  acute  and  chronic. 


220     STRUCTURAL  CHANGES  IN  PATHOLOGY  OF  PULP 

Acute  Hyperemia. — It  is  one  of  the  most  important  of  the 
pathologic  conditions  of  the  pulp,  because  it  is  one  of  the 
most  common,  and  is  often  the  first  of  a  series  which  result 
in  the  final  loss  of  the  organ.  It  is  this  condition  of  the  pulp 
which  most  commonly  calls  the  patient's  attention  to  the 
presence  of  a  carious  cavity.  The  destruction  of  the  tooth 
tissue  as  well  as  the  irritation  of  the  dentinal  fibrils  by  the 
acid  produced,  increase  the  irritability  of  the  cytoplasm  of  the 
odontoblasts,  and  the  normal  sensory  function  of  the  pulp  is 
greatly  exaggerated.  The  response  to  sudden  changes  of 
temperature  which  constitutes  the  normal  sensory  function  of 
the  pulp  is,  in  fact,  a  momentary  acute  hyperemia,  which  is 
immediately  recovered  from  by  the  return  of  the  normal 
caliber  of  the  arteries.  The  reaction  is  brought  about  by  the 
vasomotor  nerves  which  control  the  arteries.  As  soon  as 
the  artery  dilates  a  greatly  increased  amount  of  blood  is 
poured  into  the  tissue,  and  all  of  the  minute  capillaries  are 
distended  to  three  or  four  times  their  size  (Figs.  164  and 
165).  Because  of  the  semifluid  character  of  the  intercellular 
substance,  the  pressure  is  transmitted  to  the  nerves,  and  a 
sharp  lancinating  pain  is  the  result,  which  lasts  until  the  dis- 
tention of  the  bloodvessel  subsides,  which  occurs  in  a  few 
seconds  in  normal  conditions. 

When  a  tooth  is  exposed  continuously  to  sudden  changes 
of  temperature,  or  to  excessive  heat,  the  function  becomes 
greatly  exaggerated,  and  changes  which  would  ordinarily 
produce  no  effect  will  produce  acute  hyperemia.  When  the 
irritability  of  the  fibrils  and  the  odontoblasts  has  been  greatly 
increased  by  the  action  of  irritating  agents,  as  in  the  progress 
of  caries,  or  when  the  thickness  of  the  protecting  dentine 
has  been  greatly  reduced,  as  in  abrasion,  or  when  consider- 
able masses  of  gold  are  separated  from  the  pulp  only  by  a 
thin  layer  of  dentine,  the  same  conditions  result. 

In  this  stage  of  hyperemia  the  only  change  in  the  tissue 
that  can  be  observed  under  the  microscope  is  the  distention 
of  the  capillaries  and  veins,  and  as  soon  as  the  pain  has 
passed  the  tissue  returns  to  a  normal  condition. 

It  is  apparently,  therefore,  a  functional  disturbance,  due 


ACUTE  HYPEREMIA 


221 


to  the  increased  irritability  of  the  cytoplasm  of  the  fibrils, 
odontoblasts,  and  probably  also  of  the  nerve  endings.  The 
rational  treatment  for  such  conditions  is  the  removal  of  the 


Fig. 

164 

.'.'l^B 

m 

r. 

T\ 

I 

■''V  ' 

^m 

%^' 

Acute  hyperemia. 
Fig.  165 


Acute  hyperemia,  higher  power. 


222     STRUCTURAL  CHANGES  IN  PATHOLOGY  OF  PULP 

irritation  which  has  caused  the  irritabiUty,  and  the  complete 
protection  of  the  tooth  from  thermal  change,  until  the  rest 
restores  the  normal  function. 

In  order  to  observe  the  structural  changes,  the  tooth  must 
be  extracted  during  the  paroxysm  of  pain,  and  should  be 
cracked  and  dropped  at  once  into  a  fixing  fluid,  allowed  to 
remain  there  for  about  twenty-four  hours,  when  the  pulp 
can  be  removed  from  the  pulp  chamber  and  embedded  and 
sectioned.  In  this  w^ay  the  injection  of  the  bloodvessels  will 
be  preserved,  and  all  of  the  capillaries  and  veins  w^ill  be 
found  crow^ded  with  corpuscles,  and  their  distention  will  be 
proportionate  to  the  severity  of  pain  at  the  time  of  the 
extraction. 

Acute  hyperemia  has  two  possible  terminations  aside 
from  recovery:  (1)  If  often  repeated,  it  may  pass  over  into 
chronic  hyperemia;  (2)  if  severe  enough,  it  may  end  in 
infarction. 

Chronic  Hyperemia. — When  the  paroxysms  of  acute 
hyperemia  are  often  repeated  the  endothelial  cells  of  the 
bloodvessel  walls  lose  their  power  of  contractility,  and  the 
vessels  remain  permanently  dilated.  In  this  condition 
the  bloodvessels  are  dilated,  even  between  the  paroxysms  of 
pain,  and  thin  w^alls  often  shoW'  pouches  and  varicosed 
enlargements.  (See  Black,  American  System  of  Dentistry, 
vol.  i,  p.  846.)  During  the  paroxysms  of  pain,  both  red 
and  w^hite  blood  corpuscles  are  forced  through  the  blood- 
vessel walls,  and  areas  of  breaking  down  red  blood  corpus- 
cles may  be  found  in  the  tissue.  The  number  of  cellular 
elements  in  the  pulp  becomes  greatly  increased.  Whether 
this  increase  is  due  to  the  multiplication  of  the  connec- 
tive-tissue cells  of  the  pulp  or  to  the  development  into 
tissue  elements  of  w^hite  blood  corpuscles  is  a  matter  upon 
which  opinions  differ.  In  the  study  of  such  specimens 
the  author  has  been  unable  to  escape  the  feeling  that  many 
of  the  white  blood  corpuscles  develop  into  tissue  elements, 
w^hich,  however,  have  not  the  form  of  the  typical  connective- 
tissue  cells  of  the  pulp. 

Fig.  166  is  a  photograph  of  a  section  of  such  a  pulp,  and 


INFARCTION 


223 


by  comparison  with  Fig.  167  the  increase  in  the  number  of 
cellular  elements  is  very  apparent. 

Chronic  hyperemias  are  usually  followed  after  exposure 
of  the  pulp  by  inflammation  and  suppuration,  but  if  this 


Fig.  166 


Chronic  hyperemia,  showing  increase  of   cellular  elements 


is  prevented  by  the  treatment  of  the  cavity,  the  tissue  is 
likely  to  undergo  fibrous  or  other  degeneration. 

Infarction.  —  Complete  infarction  of  the  dental  pulp  is 
rare,  but  partial  conditions  are  not  uncommon.  The  con- 
dition is  comparable  to  the  conditions  that  occur  in  the  brain 


224     STRUCTURAL  CHANGES  IN  PATHOLOGY  OF  PULP 

and  other  places  supplied  by  end  arteries,  without  anasto- 
mosis, when  the  vessels  carrying  the  blood  from  the  part  are 
completely  occluded. 

The  following  clinical  picture  will  occasionally  be  encoun- 
tered with  such  a  history.  A  tooth  beginning  to  ache 
suddenly,  perhaps,  because  of  a  sudden  exposure  to  change 
of  temperature,  continues  to  ache  violently  several  hours, 
the  pain  being  described  as  acute  and  lancinating,  and  so 
severe  as  to  be  almost  intolerable.  Nothing  done  relieves 
the  symptoms  in  the  least.  Finally,  the  pain  stopped  almost 
as  suddenly  as  it  began.  The  next  morning  the  tooth  is 
more  or  less  red  in  color,  and  by  the  time  it  reaches  the 
operator  it  begins  to  turn  dark.  What  has  happened  is 
represented  in  the  following  tissue  changes.  An  extremely 
acute  hyperemia  has  occurred,  the  dilatation  of  the  arteries 
entering  the  apical  foramina  have  compressed  the  more 
delicate  walls  of  the  veins  so  as  to  occlude  them  completely, 
and  greatly  increased  blood  pressure  has  distended  all  of 
the  capillaries  and  veins,  forcing  the  red  and  white  blood 
corpuscles,  as  well  as  the  serum,  through  their  walls,  filling 
the  tissue.  Complete  stasis  has  resulted  after  a  few  hours 
in  the  death  of  all  the  tissue  elements,  at  which  time  the 
pain  stopped.  The  serum  has  dissolved  the  hemoglobin 
from  the  red  blood  corpuscles,  filtered  through  the  dentinal 
tubules,  discoloring  the  dentine  and  showing  through  the 
enamel.  Small  areas  of  partial  infarction  are  found  in  many 
specimens  after  severe  paroxysms  of  acute  hyperemia^  which 
may  be  recovered  from  entirely. 

The  severe  pain  which  occasionally  results  from  the 
application  of  arsenic  for  the  devitalization  of  pulps  is  due 
to  the  acute  hyperemia  which  is  induced.  The  removal  of 
the  arsenic  application  will  not  alleviate  the  pain,  which 
can  be  subdued  only  by  the  immediate  extirpation  of  the 
pulp. 

Inflammation. — Inflammation  of  the  pulp  occurs  ordin- 
arily only  after  exposure,  and  follows  a  chronic  suppurating 
course,  progressing  along  the  veins;  the  line  of  demarcation 
between  the   normal   and   inflammatory  areas  often  being 


PULP  NODULES  225 

quite  sharply  marked.  In  the  first  stages  the  white  cor- 
puscles are  seen  along  the  walls  of  the  vessels,  passing  through 
the  walls  into  the  tissue  in  increasing  numbers  until  the 
tissue  becomes  a  solid  mass  of  cells  and  serum  breaking 
down  into  pus.  This  progresses  until  the  entire  tissue  is 
destroyed.  There  is  the  greatest  difference  in  the  rapidity 
with  which  the  stages  follow  each  other  and  the  extent  to 
which  the  inflammation  spreads  through  the  tissues  before 
the  breaking  down  begins.  This  is  probably  due  both  to 
the  character  of  the  invading  microorganisms  and  the 
resistance  of  the  individual.  These  conditions  are  illus- 
trated in  Figs.  167,  168,  169,  170,  171,  and  172.  The  for- 
mation of  pulp  nodules  is  often  noted  in  the  deeper  part  of 
the  tissue  in  which  inflammation  is  progressing  (Fig.  169). 
Occasionally  centres  of  inflammation,  progressing  to  abscess 
formation,  are  found  within  the  substance  of  the  pulp  (Fig. 
174).  These  are  apparently  true  intrapulpal  abscesses  and 
present  the  characteristics  of  miliary  abscesses  in  any  other 
tissue. 

Degeneration. — The  embryonal  character  of  the  pulp 
tissue  renders  it  specially  susceptible  to  degenerative  changes, 
but  the  degenerative  changes  of  the  dental  pulp  have  never 
been  adequately  studied.  It  is  extremely  difficult  to  obtain 
material.  Teeth  without  histories  are  practically  useless, 
and  large  numbers  of  specimens  are  necessary. 

Pulp  Nodules. — In  the  cutting  of  large  nuqabers  of  pulps 
for  the  preparation  of  class  work  the  author  has  been  im- 
pressed by  the  frequency  with  which  hard  nodules  occur 
in  the  tissue.  These  are  apparently  of  several  varieties, 
some  of  which  are  calcified  and  others  are  not.  They  usually 
occur  in  the  coronal  portion  of  the  pulp  near  the  opening 
of  the  canals.  They  often  occur  in  specimens  in  which  the 
tissue  is  otherwise  normal.  Fig.  176  shows  a  section  with  a 
small,  almost  spherical  nodule  in  the  centre  of  the  lower 
part  of  the  coronal  portion.  For  a  number  of  sections  the 
nodule  was  cut  through  as  if  it  had  a  soft  periphery,  then  it 
took  a  nick  out  of  the  razor  and  was  pulled  out  of  the  tissue, 
the  subsequent  section  showing  the  hole  it  had  left  (Fig.  177). 
15 


226  STRUCTURAL  CHANGES  IN  PATHOLOGY  OF  PULP 

Fig.   167 


Beginning  inflanunation. 


^^"^    -  -  ^—  ^-->-o<^/<r^^^;^^^ 


■V^'''--. 


Minute  inflammatory  focus  within  the  tissues  of  the  pulp:   a,  a,  arterial  twigs; 
b,  a  nerve  bundle;  c,  collection  of  leukocytes.     (Black.) 


PULP  NODULES 


227 


Fig.  169 


Section  of   dental   pulp,  showing  the  invasion  of  the  inflammatory  process   along 
the  veins  and  the  diapedesis  of  white  blood  corpuscles. 


Fig.  17C 


Inflammation  of  the  pulp,  showing  pulp  nodules. 


228     STRUCTURAL  CHANGES  IN  PATHOLOGY  OF  PULP 


Fia.  171 


Inflammation  of  the  pulp. 
Fig.  172 


y^ 


'^^y^'- 
-^'^4^, 


'-i^-^^'r^; 


Development  of  inflammatory  tissue  elements  in  the  pulp:  a,  normal  cells; 
h,  inflammatory  elements;  c,  cells  in  process  of  division.    (xVobj.) 


PULP  NODULES 


229 


Fig.  173 


m^ 


Progressive  suppuration  of  the  pulp  of  an  incisor:  a,  healthy  tissue;  6,  odonto- 
blast laj'er,  or  membrana  eboris;  c,  inflamed  tissue,  in  which  the  veins  are  seen  to 
be  dilated;  d,  line  of  demarcation  of  the  suppurative  process;  e,  pus.  A  part  of  the 
crown  portion  of  the  pulp  had  been  destroyed  by  suppuration,  and  in  the  remaining 
portion  it  will  be  noted  how  the  pulp  is  hollowed  out,  the  process  pursuing  the 
course  of  the  veins  and  converging  to  the  centre.    (100  X,  reduced.)     (Black.) 


Fia.  174 


Tntiapulpal  abscess. 
Fig    175 


Abscess  within  the  tissue  of  the  pulp;  the  field  includes  about  one-half  of  the  little 
pocket  of  pus.    (About  250  X)      (Black.) 


PULP  NODULES 


231 


These  conditions  have  been  repeated  many  times  in  the 
cutting  of  sections.  In  connection  with  inflammatory  con- 
ditions,  nodules   are   often   found   in   the   deeper  portion. 


Fig.  176 


Pulp  nodules. 


These  are  apparently  calcoglobulin,  and  are  to  be  compared 
with  the  formation  of  phleboliths  in  the  varicosed  veins. 

The  nodules  in  the  coronal  portion  of  the  pulp  are  usually 
irregular  in  form  and  more  or  less  nodulated.    They  present 


232     STRUCTURAL  CHANGES  IN  PATHOLOGY  OF  PULP 


Fig.  178 


A  small  pulp  nodule,  as  seen  with  a  low  power,  showing  its  nodulation : 
a  represents  the  natural  size.    (15  X)      (Black.) 


Fig.  179 


Section  of  a  pulp  nodule,  showing  many  calcospherites,  as  pointed  out  by  o,  a 

(Black.) 


PULP  NODULES 


233 


an  infinite  variety  of  size,  shape,  and  number.  They  often 
contain  calcospherites  embedded  in  a  granular,  structureless, 
calcified  mass  (Figs.  178  and  179).    The  calcospherites  have 


Fig.  180 


Pulp  nodules  in  the  canal  portion  of  the  pulp.    (15  X)    (Black.) 
Fig.  181 


Nodules  in  root.     A  photomicrograph  of  the  section  from  -which  Fig.  ISO  was  drawn. 


234     \STRUCTURAL  CHANGES  IN  PATHOLOGY  OF  PULP 

a  small  point  at  the  centre  and  concentric  rings  around  it, 
but  they  usually  make  up  a  smaller  portion  of  the  nodule. 
The  nodules  in  the  root  portion  are  usually  rounded  in  out- 
line and  completely  calcified  Figs.  180  and  181). 


Fia.  182 


Dental  tumor  within  the  pulp  chamber:  A,  diagram  of  the  tooth,  with  dotted  line 
showing  the  position  of  the  section  B.  In  B  the  pulp  chamber  is  shown  in  section, 
nearly  natural  size,  showing  the  tumor  within.  C  is  an  illustration  of  the  tissue  of 
the  tumor:  a,  a,  the  primary  dentine;  b,  irregular  tubules  connecting  the  newgrowth 
with  the  primary  dentine — most  of  these  are  very  dark  and  irregular;  c,  a  calco- 
spherite  included  in  the  mass;  d,  apparently  a  bloodvessel  calcified;  e,  calcified  tissue; 
/,  a  finely  granular  mass;  g,  a  spur  of  very  transparent  dentine.  Dentinal  tubules 
appear  at  h,  h.     (Black.) 

With  the  exception  of  the  formation  of  calcoglobulin  in 
connection  with  inflammation,  the  author  has  never  seen  any 
indication  that  pulp  nodules  were  associated  with  patho- 
logic conditions.     They  are   apparently   more   common   in 


PULP  NODULES  235 

the  pulps  of  old  and  middle-aged  people  and  are  continually 
found  in  the  pulps  from  teeth  that  give  no  history  of  trouble. 
They  are  apt  to  be  found  in  mouths  where  there  has  been 
considerable  abrasion,  or  where  dentine  has  been  exposed  by 
caries;  but  they  are  just  as  apt  to  occur  in  the  teeth  that  have 
for  some  reason  escaped,  the  irritation  of  one  tooth  causing 
the  deposits  in  the  pulps  of  others  as  well  as  the  one  affected. 
There  seems  to  be  a  relationship  between  the  irritation  of 
dentinal  fibrils  and  these  formations  in  the  pulp. 

Dr.  Black  has  classified  the  hard  formations  occurring 
within  the  pulp  chamber  under  the  following  six  heads : 

1.  Secondary  dentine,  a  new  growth  of  dentine,  more  or 
less  regular  in  formation,  excited  by  abrasion,  decay,  or 
other  injury,  by  which  the  dentinal  fibrils  are  subjected  to 
irritation  at  their  distal  ends.  This  has  already  been  con- 
sidered under  the  headings  both  of  the  dentine  and  the 
pulp. 

2.  Dental  tumor  within  the  pulp  chamber;  an  erratic 
growth  of  dentine  into  the  pulp  chamber,  united  to  the  wall 
by  a  pedicle.  The  structure  is  usually  very  irregular.  These 
are  comparatively  uncommon  (Fig.  182). 

3.  Nodular  calcifications  among  but  not  of  the  pulp  tissue; 
these  are  the  irregular  nodulated  masses  so  frequently  seen 
either  as  large  or  small  pulp  stones.  They  contain  many 
calcospherites. 

4.  Interstitial  calcifications  of  the  pulp  tissue;  this  is  the 
counterpart  of  calcifications  elsewhere  in  the  body,  as  in  the 
artery  walls. 

5.  Cylindrical  calcifications  of  the  pulp,  the  tissues  of 
which  are  probably  in  a  state  of  fibrous  degeneration,  usually 
seen  in  the  pulp  canals  (the  so-called  lead  wire  pulp). 

6.  Osteodentine;  erratic  formations  showing  both  the 
lacunse  of  bone  and  dentinal  tubules. 


CHAPTER  XVII 

INTERCELLULAR  SUBSTANCES 

During  the  last  hundred  years,  knowledge  of  living  things 
and  all  thought  of  their  structure  and  function  has  entirely 
changed.  The  cell  theory  has  abundantly  established  that 
the  cell  is  the  structural  and  functional  unit  of  all  living 
objects,  both  plant  and  animal,  and  that  all  manifestations 
of  life  are  accomplished  by  the  chemical  activity  of  the 
substance  of  the  cell,  which  Huxley  long  ago  designated  as 
"The  physical  basis  of  life."  From  a  consideration  of  the 
physical  properties  of  cytoplasm,  nothing  is  more  apparent 
than  that  the  production  of  a  highly  organized  body  out 
of  it  alone  would  be  impossible.  If  the  human  body  were 
composed  entirely  of  cytoplasm  it  would  be  a  shapeless  lump 
of  jelly.  It  is  only  by  the  production  of  material  which  has 
physical  properties  of  strength  and  rigidity  through  the 
activity  of  the  cytoplasm  that  the  shape  and  function  of  a 
highly  organized  creature  is  possible.  This  is  accomplished 
through  the  metabolism  of  the  cytoplasm  more  or  less 
analogous  to  the  building  up  of  a  secretion  by  the  cells  of  a 
gland,  though  there  is  no  intention  to  suggest  any  direct 
comparison  between  the  two.  In  other  words,  all  tissues  are 
made  up  of  cells  and  intercellular  substance,  and  the  vital 
characteristics  are  given  to  the  tissue  by  the  cells,  the 
physical  characteristics  by  the  intercellular  substance.  These 
intercellular  or  extracellular  materials  possess  none  of  the 
vital  manifestations,  and  are  entirely  dependent  upon  the 
cells  for  their  formation  and  maintenance.  There  is  appar- 
ently a  constant  reaction  between  the  cell  and  the  formed 
material  which  constitutes  the  intercellular  substance,  for 
even  the  most  highly  specialized  of  intercellular  substances 
represented  by  the  dentine  matrix  changes  in  its  properties 


INTERCELLULAR  SUBSTANCES  237 

if  the  cells  are  removed.  If  the  cells  in  the  bone  matrix  are 
killed,  that  portion  of  the  tissue  becomes  necrosed  bone  and 
is  as  much  a  piece  of  foreign  matter  as  if  a  piece  of  bone 
toothbrush  handle  had  been  shot  into  the  body.  The  fibers 
of  fibrous  tissue  have  no  ability  to  grow,  to  attach  themselves 
to  any  surface,  or  even  to  maintain  their  present  form  with- 
out the  presence  of  living  cells  or  fibroblasts.  There  has 
been  a  great  deal  of  discussion  as  to  the  method  of  forma- 
tion of  intercellular  substances  by  the  cells,  and  the  nature 
of  the  reaction  occurring  between  the  cell  and  the  formed 
material  after  it  has  been  produced.  In  several  intercellular 
substances  the  material  passes  through  changes  both  of 
physical  and  of  chemical  character,  but  these  are  carried  out 
by  reaction  with  materials  formed  by  the  metabolism  of  the 
cell,  for  if  the  cells  are  removed  the  formed  material  will  not 
go  through  any  such  changes.  The  intercellular  substances, 
therefore,  while  they  are  chemically  extremely  complex, 
belong  to  the  simplest  classes  of  protein  molecules,  and  have 
no  such  complexity  of  atomic  movement  producing  conditions 
of  recurrent  unsatisfied  afiinity,  without  which  no  idea  of 
the  metabolism  of  living  c}i:oplasm  can  be  obtained.  Chem- 
ically, living  cytoplasm  may  be  roughly  viewed  as  con- 
stantly undergoing  chemical  changes  which  are  almost 
infinitely  complex,  and  by  means  of  which  simpler  substances 
are  acted  upon  and  built  into  its  own  molecule.  Complex 
combinations  are  thrown  off  as  products  of  its  metabolism, 
and  simpler  substances  are  formed  as  decomposition  products, 
or  waste  materials.  Dr.  Brooks  often  used  to  say  in  his 
lectures  that  the  most  striking  characteristic  of  living 
things  was  their  ability  to  react  upon  their  environment  in 
such  a  way  as  to  become  better  and  better  suited  to  it. 
When  living  cytoplasm  which  is  soft  and  without  the  physical 
properties  of  strength  and  rigiditv  requires  protection  from 
physical  influences,  substances  possessing  these  qualities 
are  produced  by  it.  Intercellular  substances,  therefore,  were 
apparently  formed  by  the  cytoplasm  in  response  to  physical 
conditions  of  its  environment,  and  are  one  of  the  phases  of 
adaptation. 


238  INTERCELLULAR  SUBSTANCES 

In  the  higher  forms  of  animal  life  the  class  of  tissues 
which  have  produced  these  formed  materials,  for  the  purpose 
of  support,  rigidity,  and  connection,  are  called  the  connective 
or  supporting  tissue.  The  formed  materials  are  of  two 
classes — those  which  are  to  connect  associated  and  dependent 
parts,  and  those  which  give  rigidity  and  protection.  The 
fibrous  tissues  are  of  the  first  class,  and  are  made  up  of 
materials  possessing  strength  and  elasticity.  The  bone  and 
cartilage  belong  to  the  second  class,  and  give  strength  and 
rigidity.  The  first  sustain  pulling  stress,  the  latter  shearing 
or  bending  stress,  though  both  possess  a  certain  amount 
of  each. 

Adaptability  and  the  greatest  range  of  variation  are  most 
striking  characteristics  of  connective  tissue  which  develop 
and  change  to  meet  all  kinds  of  requirements  of  both  mechan- 
ical and  physical  environment  to  which  they  are  subjected. 
These  variations  are  produced  by  the  production  of  increased 
amount  of  the  intercellular  material,  its  destruction,  or  the 
change  of  its  character,  under  the  influence  of  the  cells  of 
the  tissue.  No  tissue  responds  more  quickly  to  the  demands 
made  upon  it  by  development.  When  the  muscles  grow 
larger  and  stronger  by  development,  the  tendons  and  the 
bones  to  which  they  are  attached  change  as  quickly  and  in 
proportion.  From  the  appearance  of  the  skeleton  the  experi- 
enced anatomist  can  picture  very  accurately  the  muscular 
development  of  the  individual  to  whom  it  belonged. 

The  cell  wall  of  plants  may  be  used  as  one  of  the  simplest 
examples  of  supporting  tissue.  In  this  case  each  cell,  in 
addition  to  its  other  functions,  produces  its  own  supporting 
substance.  These  may  be  observed  in  the  cells  of  a  growing 
root  tip.  Plant  an  onion,  by  selecting  one  larger  than  a 
small  glass,  fill  the  glass  with  water,  and  place  the  bulb  on  it. 
If  this  is  placed  in  a  sunny  window,  in  a  few  hours  little 
rootlets  will  be  seen  stretching  down  into  the  water.  The 
rootlets  of  a  sprouting  chestnut  also  make  very  good  material 
(Fig.  183).  If  these  are  embedded  in  paraffin,  the  develop- 
ment of  the  cells  and  the  formation  of  their  supporting  walls 
can  be  observed.    The  young  cells  near  the  tip  will  be  found 


INTERCELLULAR  SUBSTANCES 


239 


to  be  a  mass  of  granular  protoplasm,  with  a  large  nucleus 
in  the  centre,  and  a  thin  wall  of  cellulose  which  is  the  cell 
organ  of  support.  As  the  cell  increases  in  size,  vacuoles 
appear  in  the  cytoplasm  which  become  larger  and  larger. 
These  vacuoles  are  filled  with  watery  fluid  which  is  not  a 
part  of  the  cytoplasm.  If  the  cell  remained  a  solid  mass  of 
cytoplasm,  an  enormous  amount  of  food  material  would  be 
required,  which  would  be  out  of  all  proportion  to  the  work 
which  the  cell  is  to  perform.    The  vacuoles  increase  in  size 

Fig.  183 


fp^rr 

K, 

1 

K 

1 

J 

;> 

'W 

^. 

^^$ 

t§M 

4 

Yf 

l^^ 

jtaWia^^ 

Cells  from  the  growing  tip  of  a  chestnut  seedling. 


with  the  growth  of  the  cell  until  there  is  a  rim  of  cytoplasm 
in  contact  with  the  cell  wall,  and  a  central  mass  of  cytoplasm 
surrounding  the  nucleus  and  connected  with  that  at  the 
periphery  by  fine  threads.  In  still  further  growth  these 
threads  are  broken,  the  nucleus  is  pushed  to  one  side,  and 
the  whole  central  portion  becomes  one  huge  vacuole.  There 
is  now  a  cell  wall,  with  a  layer  of  cytoplasm  covering  its 
inner  surface,  which  is  kept  in  reaction  with  the  nucleus  by 
streaming  around  and  around.  This  flowing  of  the  cytoplasm 
in  plant  cells  may  be  easily  observed  in  the  dehcate  stamen 


240  INTERCELLULAR  SUBSTANCES 

hairs  of  the  ordinary  Spiderwort,  or  in  the  cells  of  the 
water  plants  Chara  or  Nitella,  which  are  easily  found  in 
most  ponds.  In  this  example  it  is  seen  that  the  cytoplasm 
remains  in  contact  with  the  formed  material  which  it  produces 
for  support,  and  that  it  is  only  sufficient  in  amount  to  form 
and  maintain  this  material. 

In  general  histology  it  has  already  been  noted  that  the 
cells  of  connective  tissue  are  very  similar,  and  that  the 
tissues  differ  chiefly  in  the  character  and  arrangement  of 
the  intercellular  substances.  It  has  also  been  emphasized 
that  the  connective  tissues  all  originate  from  a  common 
form  of  embryonal  connective  tissue,  or  mesenchyme,  and 
change  from  one  form  to  another  in  development.  These 
mutations  of  the  connective  tissues  are  its  most  striking 
characteristic,  and  must  be  clearly  grasped  if  the  bone,  as 
an  organ  of  support,  is  to  be  understood.  For  instance," 
embryonal  connective  tissue  is  transformed  into  fibrous 
tissue;  fibrous  tissue  becomes  arranged  in  a  definite  mem- 
brane, and  is  transformed  into  cartilage,  which  is  again 
removed  and  transformed  into  bone.  All  these  changes  take 
place  to  meet  the  requirement  of  mechanical  conditions  and 
influences. 

If  the  subcutaneous  tissue  of  an  embryo  be  examined  in  sec- 
tions (Figs.  184  to  199)  the  cells  will  be  found  to  be  irregular 
masses  of  cytoplasm  with  a  nucleus  in  the  central  portion, 
and  fine  projections  stretching  out  in  all  directions  through 
an  almost  structureless  intercellular  substance.  The  fine  pro- 
jections of  the  cytoplasm  meet  those  of  the  adjoining  cells 
and  form  a  network  holding  everything  together.  Because  of 
the  nature  of  cytoplasm,  however,  these  possess  very  little 
strength,  and  very  soon  fine  thread-like  fibers  are  found 
appearing  in  the  intercellular  substance  in  contact  with 
cells.  These  unite  with  each  other,  forming  continuous 
fibers,  and  very  soon  a  strong  netw^ork  is  produced  which 
is  entirely  dependent  upon  the  cytoplasm  of  the  cell  which 
has  formed  and  maintains  it.  If  this  tissue  is  now  subjected 
to  pressure  and  strain,  the  cells  become  flattened  out  and 
squeezed   between  the  bundles  of  fibers,   which  take  on 


INTERCELLULAR  SUBSTANCES 


241 


parallel  directions,  and  so  a  tendon  is  formed.     A  tendon 
must  be  considered  as  a  highly  specialized  form  of  connective 


Fig.  184 


Fig.  185 


--KK<', 


^^:^- 


-^ 


^^i-T-S 


Embn,'onal  connective  tissue  in  an  early  The  same,   a  little  more   developed, 

stage  of  development,  showing  the  cellular      showing  the   cellular  elements    length- 
elements   embedded   in    the    ground    sub-      ening  in  a  common  direction, 
stance. 

Fig.  186 


^--^"-ir^^^^---  - 


The  cells  developed  in  spindle  forms,  fibroblasts  with  long  filaments  extending 
from  either  end. 


Fig.  187 


The  developed  white  fibrous  tissue. 


tissue,  arranged  to  supply  tensile  strength.    The  degree  of 
specialization  of  the  tissue  is  judged  by  the  extent  to  which 
16 


242 


INTERCELLULAR  SUBSTANCES 


its  characteristic  features  are  developed,  either  in  quantity 
or  quality.     In  the  tendon  the  fine  strong  fibers  have  l)een 


Fig.  188 


^ 


Older  white  fibrous  tissue,  in  which  the  cells  are  no  longer  seen,  and  showing 
the  wave  like  course  of  the  fibers. 


Fig    189 


Coarse  white  fibers,  made  up  of  bundles  of  the  fine  fibers,  and  showing  the  mode 
of  division  by  splitting  off  of  a  portion  of  the  fibers  of  the  bundle. 


Fig.  190 


Coarse  fiber  breaking  up  into  fine  fibers. 


gathered  into  bundles;  a  round  nucleus  would  occupy  too 
much  space.    It  has,  therefore,  become  elongated  and  more 


INTERCELLULAR  SUBSTANCES 


243 


or  less  rod-shaped,  and  the  cytoplasm  has  been  squeezed 
out  into  thin   leaf-like   projections   between  the  bundles. 


Fig.  191 


Cross-sections  of  coarse  fibers,  showing  some  of  their  various  forms. 


Fig.  192 


Fig.  193 


& 


Reticular  or  elastic  fibers,  showing  the  mode  of  division 
and  the  multipolar,  or  irregular,  star  forms  of  the  cells 
at  the  divisions. 

Fig.  194 


^•/ 


^  o 

Cross-sections  of 
the  reticular  fibers, 
showing  some  of 
their   forms. 


Connective-tissue  cells  from  which  reticular  fibers  are  developed. 


Each  cell  is  in  contact  with  several  fibers,  and  each  fiber  in 
contact  with  the  cytoplasm  of  cells  which  have  produced 
them. 


244 


INTERCELLULAR  SUBSTANCES 


Fig    195 


Fig.  196 


Network  of  elastic  fibers  from  the  point  Network  of  elastic   fibers   teased  out 

of  reflection  of  the  mucous  membrane  of       from  elastic   tendon,   and  showing    the 
the  lip  from  the  gums.  usual  mode  of  division. 


Fig.  197 


/ 


^s-^ 


^yf^S' 


Elastic  fibers,  showing  their  disposition  to  curl  up  when  cut  or  broken. 


INTERCELLULAR  SUBSTANCES  245 

It  must  be  supposed  that  there  is  a  constant  reaction 
between  the  substance  of  the  formed  material  and  materials 
produced  by  the  metabolism  of  the  c^i:oplasm.  In  patho- 
logic conditions  the  metabolism  of  the  cytoplasm  is  dis- 
turbed, and  there  is  a  consequent  change  in  the  quality  of 
the  fibers.  So  in  some  pathologic  conditions  a  relaxation 
and  loss  of  tone  is  found  in  tendons  and  ligaments.  In 
inflammations  of  the  gingivae  the  fibers  become  relaxed  and 
stretched,  so  that  the  gingivse  are  everted,  but  return  to 
their  normal  condition  when  the  pathologic  condition  has 
subsided,  and  the  cells  regain  their  normal  metabolism. 

Fig    198  Fig.  199 

■:-:>^'i-~/ir<''  .  -  r.  ,,   '^ '  ■',■■ 

fc?:.-;  .-  ::  .\.:::-:^- :-, --^^ '.:'.::-..  r-^.---..f^^^-& 
Cross-sections  of  elastic  fibers, 
showing   their   forms    as   seen   in  Tissue  of   the   dental  pulp,  in  which  the  de- 

a   group   passing  between   coarse        velopment  of   the  cells  is  not  followed  by  any 
white   fibers.  considerable  formation  of  fibers. 

To  sum  up  w^hat  has  been  said,  it  is  apparent  that  both 
phylogenetically  and  ontogenetically,  intercellular  substances 
have  been  produced  and  are  maintained  by  cells  in  response 
to  mechanical  influences  and  to  meet  mechanical  conditions. 
In  all  higher  animals  certain  tissues,  the  connective  tissues, 
have  been  set  apart  for  this  purpose,  and  the  cells  have  been 
specialized  to  respond  to  mechanical  stimuli  and  develop 
an  intercellular  substance  adapted  to  the  condition.  This 
makes  the  supposition  necessary  that  an  embryonal  connec- 
tive-tissue cell  may  develop  into  any  specialized  form  and 
that  the  kind  of  cell  into  which  it  develops  will  be  determined 
by  the  character  of  mechanical  stimuli  which  it  receives. 
Just  as  the  epithelial  cells  have  been  specialized  to  respond 
to  the  environments  of  light  stimuli,  vibration  of  the  air, 
pressure,  and  chemical  action  which  connect  the  organism 
with   its   environment,    connective-tissue   cells   have   been 


246  INTERCELLULAR  SUBSTANCES 

specialized  to  respond  to  mechanical  stimuli,  by  the  pro- 
duction of  formed  materials  adapted  to  the  mechanical 
conditions.  These  conceptions  are  fundamental  to  an 
understanding  of  bone  structure  and  growth,  and  the  muta- 
tions of  connective  tissue  in  general. 

In  no  branch  of  histology  is  a  clear  conception  of  inter- 
iellular  substances  and  the  relation  of  cells  to  them  as 
mportant  as  in  the  study  of  the  teeth  and  their  associated 
structures.  Caries  cannot  be  understood  unless  these 
fundamental  ideas  have  been  appreciated,  and  many  state- 
ments in  dental  literature  would  never  have  appeared  if 
the  nature  of  intercellular  substance  and  the  relation  of 
cytoplasm  to  it  had  been  understood. 


CHAPTER  XVIII 

BONE 

Defimtion. — Bone  may  be  defined  as  a  connective  tissue 
whose  intercellular  substance  is  calcified  and  arranged  in 
layers  around  nutrient  canals  or  spaces.  The  cells  are 
placed  in  cavities,  lacunae,  between  the  layers,  and  receive 
their  nourishment  through  very  minute  channels,  canal- 
iculi,  which  radiate  from  them  and  penetrate  the  layers. 

STRUCTURAL  ELEMENTS 

The  structural  elements  of  bone  are : 

1.  Bone  matrix,  or  intercellular  substance,  which  is  always 
arranged  in  layers  or  lamellte. 

2.  The  bone  cells  or  bone  corpuscles  which  are  embedded 
in  the  matrix  between  its  layers. 

3.  Lacunae,  or  the  spaces  in  which  the  cells  are  found. 

4.  CanalicuH,  or  the  channels  through  the  matrix  by 
which  the  embedded  cells  receive  nourishment. 

Bone  Matrix. —  The  bone  matrix  is  composed  of  a  dense 
organic  basis  of  ultimately  fibrous  character  which  yields 
gelatin  upon  boiling  with  water.  With  this  inorganic  salts 
are  combined  in  a  weak  chemical  union,  forming  the  hard 
substance  of  bone.  By  treatment  with  acids  the  inorganic 
salts  can  be  removed,  leaving  the  organic  basis  which  retains 
the  form  of  the  tissue.  In  this  condition  the  rigidity  of  the 
bone  is  destroyed.  On  the  other  hand,  by  calcining  at  red 
heat  the  organic  basis  can  be  removed,  leaving  the  inorganic 
substances  which  retain  the  form  of  the  tissue.  In  forma- 
tion the  organic  basis  is  apparently  formed  first,  and  then 
the  salts  of  lime  are  combined  with  it,  through  the  agency 
of  the  formative  cells,  or  osteoblasts. 


248 


BONE 


Bone  Corpuscles.  —  Bone  corpuscles  are  the  cells  lying  in 
the  laciuiie.  Each  cell  contains  a  single  well-defined  nucleus, 
lying  in  the  centre  of  a  granular  cytoplasm.  The  cell  appar- 
ently completely  occupies  the  lacunae,  and  from  the  central 
mass   fine   projections   of   cytoplasm   extend   through   the 

Fig.  200 


From  a  section  through  the  bone  of  a  roebuck.  The  lacunae  are  seen  from 
above,  and  are  filled  with  coloring  matter.  In  places  small  dots  are  visible,  which 
represent  the  cross-sections  of  bone  canaliculi.     (850  X)     (Szymonowicz.) 


canaliculi,  which  brings  the  bone  corpuscles  in  intimate 
relation  with  certain  area  of  bone  matrix.  The  processes  of 
one  cell  anastomose  with  those  of  its  neighbors  through  the 
canaliculi,  so  that  there  is  a  continuous  network  of  living 
cytoplasm  throughout  the  matrix. 


CANALICULI 


249 


Lacunae.  —  The  lacunae  are  flat  oval  spaces  about  20 
microns  long,  10  microns  wide,  and  5  or  6  microns  thick. 
Their  shape,  therefore,  in  sections  depends  upon  the  way  in 
which  they  are  cut  as  illustrated  in  Figs.  200  and  201.    When 


Fig.  201 


From  a  section  through  the  bone  of  a  roebuck.     The  lacume  are  seen  from  the  side. 
(850  X)      (Szymonowicz.) 


cut  lengthwise  they  would  appear  as  about  20  microns  long 
and  6  wide  in  profile,  or  as  about  20  microns  long  and  10 
wide  when  seen  from  above. 

Canaliculi. — These  radiate  from  the  lacunae  in  all  direc- 
tions, opening  into  them  by  larger  channels  which  branch 


250  BONE 

and  divide,  becoming  smaller  as  they  pass  farther  into  the 
matrix.  They  anastomose  freely  with  those  from  adjoining 
lacunae. 

THE  VARIETIES   OF  BONE 

There  are  three  varieties  of  bone  differing  in  the  arrange- 
ment of  these  structural  elements.  These  are  subperiosteal, 
Haversian  system,  and  cancellous  bone. 

Subperiosteal  Bone. — This  form  of  bone  must  be  regarded 
as  primarily  a  formative  arrangement  and  more  or  less 
transitory,  in  which  the  layers  are  arranged  parallel  with  the 
surface,  and  under  a  formative  membrane.  It  contains 
canals  (Volkmann's  canals)  with  bloodvessels  (Fig.  202),  con- 
nective tissue,  etc.  These  penetrate  the  layers  which  are 
never  arranged  concentrically  around  them.  It  is  always 
thin,  that  is,  composed  of  comparatively  few  layers,  and 
when  a  considerable  thickness  is  formed  it  is  cut  out  from 
within  by  absorptions  beginning  in  the  canals,  and  bone  is 
rebuilt  with  layers  arranged  concentrically  around  the  chan- 
nels formed.  In  this  way  subperiosteal  bone  is  converted 
into  the  second  form. 

Haversian  System  Bone.^In  this  variety  the  lamellae  are 
arranged  concentrically  around  canals  which  contain  blood- 
vessels, nerves,  and  embryonal  connective  tissue,  and  from 
which  the  cells  in  the  lacunae  are  nourished  (Fig.  203).  These 
canals  are,  in  general,  parallel  with  the  surface  or  the  long 
axis  of  the  bone  and  anastomose  with  each  other.  A  canal 
with  the  layers  arranged  around  it  constitute  an  Haversian 
system.  Between  the  Haversian  systems  are  remains  of 
the  subperiosteal  layers  (insterstitial  lamellae)  that  were  left 
by  the  absorption,  and  for  that  reason  have  been  called 
fundamental  lamellae.  They  have  also  been  called  ground 
lamellae.  Haversian  system  bone  is  often  called  compact 
bone,  and  makes  up  the  greater  part  of  the  shafts  of  the 
long  bone,  and  the  plates  of  the  flat  ones.  It  is  never  allowed 
to  become  greater  in  thickness  than  is  necessary  for  strength, 
and  when  sufficient  thickness  has  been  formed,  the  deeper 


CANCELLOUS  BONE 


251 


part  is  cut  out  by  absorptions  in  the  Haversian  canals,  con- 
verting them  into  large  irregular  spaces.  The  formation  of 
a  few  layers  around  these  spaces  transforms  the  second  type 
into  the  third  or  cancellous  bone. 


Ftg.  202 


Fig.  203 


Subperiosteal  bone,  showing 
Volkmann's  canals. 


Haversian  system  bone; 
a,  Haversian  canals. 


Cancellous  Bone. — In  this  variety  the  lamellae  are  arranged 
in  delicate  plates  surrounding  large,  irregular  nutrient  or 
marrow  spaces.  These  are  filled  by  embryonal  connective 
tissue  and  contain  bloodvessels  and  nerves.  The  plates  of 
cancellous  bone  are  not  arranged  at  haphazard,  as  might  be 
supposed  from  a  casual  observation  of  sections,  but  are 
disposed  in  definite  arrangement,  which  is  determined  by 
the  directions  of  stress  on  the  compact  bone  which  they 


252  BONE 

support.  (See  illustrations  in  Chapter  XVIII.)  They  are 
not  permanent  and  unchanging,  but  are  continually  being 
rebuilt  in  new  directions,  in  response  to  the  mechanical  con- 
ditions to  which  the  bone  as  a  supporting  organ  is  subjected. 


THE  ARRANGEMENT  OF  BONE 

Compact  Bone. — A  knowledge  of  the  structural  elements 
of  bone  can  best  be  obtained  by  the  study  of  sections 
ground  from  the  shaft  of  a  long  bone.  An  old  dry  bone 
should  be  sawed  across,  near  the  middle  of  the  shaft,  in 
two  places,  so  as  to  cut  out  a  ring  about  a  quarter  of 
an  inch  thick.  Then  saw  the  ring  through  in  two  places 
with  an  arc  of  about  a  quarter  of  an  inch  on  the  outer 
surface.  From  this  two  slices  should  be  sawed  out,  one 
transverse  to  the  long  axis  of  the  bone,  the  other  parallel 
with  it.  These  are  ground  to  not  more  than  8  or  10  microns 
in  thickness  and  mounted  in  hard  balsam.  From  a  study  of 
these  two  the  arrangement  of  the  lamellae,  and  the  shape 
and  character  of  the  lacunse  can  be  made  out.  Upon  the 
outer  surface  of  the  transverse  section  will  be  found  a  larger 
or  a  smaller  number  of  layers  of  subperiosteal  bone  which 
encircle  the  shaft,  and  consequently  are  called  the  circum- 
ferential lamellae.  The  number  of  these  layers  will  depend 
upon  the  position  from  which  the  section  is  taken,  and  the 
age  of  the  bone.  If  the  bone  is  increasing  in  circumference 
at  the  point  from  which  the  section  is  cut,  there  will  be  a 
considerable  number  of  layers,  and  they  will  be  easily  seen. 
If  the  bone  has  been  growing  smaller  in  circumference  at  the 
point,  there  will  be  very  little  of  subperiosteal  bone,  and  it  will 
be  comparatively  hard  to  recognize.  The  greatest  part  of 
the  section  will  be  made  up  of  Haversian  systems,  in  which 
from  two  to  three  to  five  or  six  layers  are  arranged  around 
an  Haversian  canal.  The  lacunae  appear  as  irregularly  oval 
spaces  about  5  or  6  microns  across  and  15  to  20  microns  in 
length.  From  them  a  great  many  minute  canals  radiate 
through  the  matrix  both  toward  the  Haversian  canal  and 


PLATE   X 


K  -^ 


f: 


^     if.    V- 


X   W 


■    "-  ^  V  "^  "^      *^»  -       '      ^ 


*    1    \ 


<^ 


M 


-4-rf 


^  X    - 


i 


i 


s 


^  ^ ,  f 


^  < 


V 


.'     .A. 


< 


Froni  a  Ground  Cross-section  of  the   Diaphysis  of  the 
Human   Metatarsus.     (Szynionowicz. ) 

a,  outer  ground  lamellae;  ;>,  inner  ground  lan:iellae;  c,  Haversian  laniellae; 
d,  interstitial  laniellae.  All  canals  and  bone  cavities  are  filled  with  coloring 
nnatter  and  appear  black.      (90  X) 


COMPACT  BONE  253 

away  from  it.  The  character  of  these  canaliculi  can  only  be 
appreciated  by  seeing  them.  They  are  filled  in  life  by  pro- 
jections of  the  protoplasm  of  the  bone  corpuscles.  They  are 
suggestive  of  the  rootlets  of  plants  running  through  soil,  and 
as  in  that  case  the  rootlets  are  absorbing  material  from  the 
soil  and  reacting  with  it,  in  this  case  the  protoplasmic  con- 
tents of  the  canaliculi  is  reacting  with  the  matrix,  maintain- 
ing its  quality.  The  portion  of  matrix  through  which  the 
canaliculi  from  one  lacunae  extend  belongs  to  the  bone 
corpuscles  which  occupies  the  lacunse,  as  will  be  seen  later. 
These  cells  have  been  enclosed  in  the  matrix  which  they 
have  formed.  Between  the  Haversian  systems  will  be  found 
a  few  layers  of  interstitial  or  fundamental  lamellae.  They 
are  the  remains  of  layers  which  were  formed  under  the 
periosteum  and  were  not  entirely  destroyed  when  it  was 
replaced  by  Haversian  systems  (Plate  X).  The  amount  of 
interstitial  lamellae  varies  greatly  in  different  specimens,  as 
will  be  seen  by  comparing  figures. 

The  Haversian  canals  anastomose  with  each  other;  this 
will  be  seen  in  many  specimens.  Many  Haversian  sj^stems 
will  be  found  imperfect  in  form,  as,  for  instance,  those  shown 
in  Plate  X.  This  means  that  after  these  systems  were  com- 
pleted, absorptions  occurred  in  a  neighboring  canal  which 
attacked  the  layers  of  the  system,  and  later  a  new  system 
was  formed  in  this  space  by  the  deposit  of  concentric  lamellae. 
While  bone  is  thought  of  as  a  hard  and  fixed  tissue,  it  is  con- 
tinually being  built  and  rebuilt  in  this  way.  It  is  onty  by 
the  understanding  of  these  possibilities  that  we  get  the  ideas 
that  bone,  while  hard  and  rigid,  is  a  plastic  tissue  and  is  con- 
tinually being  moulded  by  mechanical  conditions  to  which 
it  is  subjected. 

It  will  be  seen  also  that  the  arrangement  of  the  lamellae 
becomes  a  record  of  the  changes  that  have  occurred  in  the 
formation  of  the  tissue.  The  inner  boundary  of  the  section 
next  to  the  marrow  cavity  will  show  a  few  layers  parallel 
with  the  surface.  These  are  known  as  the  inner  circum- 
ferential lamellae.  It  is  a  mistake,  however,  to  think  of 
them  as  surrounding  the  marrow  cavity  in  the  same  sense  as 


254  BONE 

the  outer  circumferential  lamellse  surround  the  bone.  If 
the  section  has  been  cut  at  a  little  distance  from  the  centre 
of  the  shaft,  it  will  have  been  noted  that  the  marrow  cavity 
is  penetrated  by  very  delicate  spicules,  and  that  in  fact  the 
marrow  cavity  is  produced  by  the  spaces  of  cancellous  bone, 
becoming  larger  and  larger  until  they  become  one  continuous 
space.  The  inner  circumferential  lamellae  are  therefore  the 
layers  which  have  been  formed  around  an  enlarged  nutrient 
or  marrow  space. 

Cancellous  Bone. — The  cancellous  bone  can  best  be  studied 
in  decalcified  sections.  A  field  from  the  central  portion  of  a 
flat  bone  will  show  its  typical  arrangement.  It  is  made  up 
of  delicate  flattened  spicules  surrounding  larger  or  smaller 
irregular  spaces  which  connect  with  each  other  very  freely. 
Each  spicule  is  composed  of  a  few  lamellae  which  are  arranged 
around  the  space.  The  structure  of  the  spicules  often 
becomes  complicated  by  absorptions  and  rebuildings  which 
have  occurred  to  change  their  direction.  The  tissue  which 
fills  the  spaces  is  a  delicate,  embrj^onal  connective  tissue 
in  which  osteoblasts  and  osteoclasts  appear  in  response  to 
mechanical  conditions.  It  is  richly  supplied  with  blood- 
vessels, nerves,  and  lymphatics.  The  lacunae  and  canal iculi 
are  in  no  respect  different  from  those  of  the  Haversian 
system  and  subperiosteal  bone. 


CHAPTER  XIX 
BONE  FORMATION  AND  GROWTH 

Bone  is  one  of  the  latest  tissues  to  be  formed,  and  is 
always  developed  from  an  antecedent  connective  tissue  of 
less  specialized  character.  According  to  the  character  of 
the  antecedent  tissue,  bone  formation  is  of  two  varieties — 
the  formation  from  cartilage,  or  endochondral  bone  forma- 
tion, and  that  from  fibrous  connective  tissue,  without  the 
intervention  of  cartilage,  or  endomemhranoiis  bone  formation. 

Endochondrial  Bone  Formation. — ^All  of  the  bones  of  the  endo- 
skeleton  are  preformed  in  cartilage.  The  transformation  of 
cartilage  into  bone  is  rather  a  substitution  than  a  transfor- 
mation, for  the  original  tissue  is  destroyed  in  the  process, 
aiKl  a  new  and  more  highly  specialized  one  substituted  for  it. 

Before  ossification  begins  the  cartilage  has  taken  on  the 
general  form  of  the  bone  and  is  covered  by  a  definite  peri- 
chondrium. Ossification  begins  at  separate  centres  and 
progresses  through  the  cartilage,  but  the  separate  centres 
do  not  unite  until  the  bone  is  about  fully  formed.  In  the 
long  bone  there  are  usually  three  centres — one  near  the 
centre  of  the  shaft,  forming  the  hypophysis,  and  one  near 
either  end,  forming  the  epiphysis.  These  remain  separated 
by  a  layer  of  cartilage  until  the  length  of  the  bone  has  been 
fully  formed. 

The  first  indication  of  the  transformation  of  cartilage 
into  bone  is  an  increase  in  the  size  of  the  lacunae  and  in 
the  amount  of  cartilage  matrix,  which  also  shows  changes 
in  character,  having  lime  salts  deposited  in  it.  The  car- 
tilage c^lls  enlarge  and  show  signs  of  degeneration,  the 
lacunse  become  arranged  in  rows,  and  as  they  increase  in 
size,  more  in  the  direction  parallel  with  the  axis  of  the 
cartilage,  the  amount  of  matrix  separating  them  is  reduced. 


256 


BONE  FORMATION  AND  GROWTH 


By  this  time  the  perichondrium,  on  the  surface  of  the  car- 
tilage opposite  to  the  centre,  has  developed  osteoblasts  which 
begin  the  formation  of  subperiosteal  lamellse  upon  the  sur- 


FiG.  204 


Hyaline 
cartilage 


Area  of 
calcification 


).',.}        Osteogeneiic 
tissue 


Perichondral 
"*"""         bone 


Capsules  containing 
•many  cartilage  cells 


From  a  longitudinal  section  of  a  finger  of  a  three-and-a-half-months  human 
embryo.  Two-thirds  of  the  second  phalanx  is  represented.  At  X  a  periosteal 
bud  is  to  be  seen.     (85  X)     (Szymonowicz  ) 


face  of  the  cartilage,  and  the  perichondrium  is  transformed 
into  periosteum.  Opposite  the  centre  osteoclasts  appear, 
cutting  into  the  cartilage,  followed  by  buds  of  embryonal 
tissue.     The  osteoclasts  dissolve  away  the  remains  of  the 


EXDOCHOXDRIAL  BOXE  FORMATION 


257 


cartilage  matrix,  opening  up  the  spaces  between  the  lacunae 
and  converting  the  rows  of  lacunae  into  irregular  channels 
or  primary  marrow  spaces.  Upon  the  spicules  of  calcified 
cartilage  matrix,  osteoblasts  arrange  themselves  and  begin 
to  lay  down  lamelhie  of  bone.    These  changes  progress  from 


Fig.  205 


Cartilage  cell 


The  place  marked    X  in  the  preceding  figure  with  stronger  magnification. 
(185  X)      (Szymonowicz). 


the  centre  in  both  directions,  and  all  stages,  from  the  typical 
hyaline  cartilage  to  the  formation  of  bone,  may  be  seen  in 
one  section.  These  stages  are  illustrated  by  Figs.  204,  205, 
and  206. 

From  now  on  the  bone  grows  by  progressive  transforma- 
tion of  cartilage  and  by  the  growth  of  bone  under  the  peri- 
osteum, which  will  be  considered  under  bone  groT\i:h. 
17 


258 


BONE  FORMATION  AND  GROWTH 


Endomembranous  Bone  Formation. — The  bones  which  are 
not  preformed  in  cartilage  are  formed  directly  from  fibrous 


Fig.  206 


Periostenm^^^] 


Periosteal  bntl 


Blood-i'essris 


filled  witJi^: 
blood^ 


Calcified    |/'/^ij'r*  '  /JJ 
cartilage'*^'  '*'     " 


Enlarged 
cartilage 
cells 


From  longitudinal  section  of  a  finger  of  a  four  months  embryo.     Only  the  diaphysis 
of  the  second  phalanx  is  represented.    (85  X)    (Szymonowicz.) 


tissue.  This  is  well  illustrated  in  the  mandible.  In  the 
region  of  Meckel's  cartilage  and  between  it  and  the  develop- 
ing tooth  germs  the  mesenchyme  begins  to  show  signs  of 


ENDOMEMBRANOUS  BONE  FORMATION 


259 


specialization.  Delicate  fibers  appear  in  the  intercellular 
substance.  Along  these  the  connective-tissue  cells  arrange 
themselves,  and,  taking  on  the  form  of  osteoblasts,  begin  to 


Fig.  207 


i3 


C    <9\0, 


^'.^>'^,  N.  ^ 


^'^-v.- 


-^  -^( 


_Primar\' 
marrow 
space. 


Osseous 

tissue. 


Section  through  the  lower  jaw  of  an  embr>-o  sheep  (decalcified  with  picric  acid). 
At  a  and  immediately  below  are  seen  the  fibers  of  a  primitive  marrow  cavity  Ij^ing 
close  together  and  engaged  in  the  formation  of  the  ground  substance  of  the  bone, 
while  the  cells  of  the  marrow  cavity,  with  their  processes,  arrange  themselves  on 
either  side  of  the  newly  formed  lamella  and  functionate  as  osteoblasts.  (Bohm, 
Davidoff,  Huber.)     (300  X) 

lay  down  bone  lamellae  (Fig.  207).  These  stretch  out  through 
the  mesenchyme,  forming  a  network  of  delicate  spicules, 
until  they  surround  Meckel's  cartilage,  and  grow  up  to  the 


260  BONE  FORMATION  AND  GROWTH 

buccal  and  the  lingual  of  the  tooth  germ.  As  soon  as  this 
network  of  bone  lamellee  containing  embryonal  connective 
tissue,  in  its  primary  marrow  spaces,  begins  to  take  on 
definite  form,  there  is  a  specialization  of  the  mesenchyme 
surrounding  it,  developing  into  fibrous  tissue  which  becomes 
a  periosteum.  From  this  time  onward  the  formation  of  bone 
progresses,  as  will  be  described  under  the  growth  of  bone. 

Bone  Growth. — If  sections  are  cut  transversely  through 
the  shaft  of  a  long  bone  from  a  fetus,  the  surface  will  be 
found  to  be  covered  by  a  well-formed  periosteum,  which  is 
actively  laying  down  layers  of  subperiosteal  bone.  The 
central  portion  of  the  bone  is  made  up  of  a  network  of 
spicules  surrounding  primary  marrow  spaces,  there  being 
no  true  marrow  cavity.  The  formation  of  the  subperiosteal 
laj^ers  does  not  progress  at  a  uniform  rate  at  all  points*  on 
the  circumference,  but  they  are  piled  up  at  certain  points 
forming  longitudinal  ridges  with  grooves  between  them. 
These  grooves  become  arched  across,  enclosing  part  of  the 
connective  tissue  of  the  inner  layer  of  the  periosteum,  and 
contain  bloodvessels  and  nerves.  Soon  after  these  spaces 
are  enclosed  absorptions  begin  in  their  walls,  destroying  a 
large  part  of  the  subperiosteal  lamellae  and  forming  primary 
marrow  spaces.  As  soon  as  these  spaces  have  reached  a 
certain  size  the  absorptions  stop,  and  osteoblasts  appear 
upon  the  wall  of  the  space  and  begin  to  lay  down  lamellae 
upon  its  circumference,  until  an  Haversian  system  has  been 
produced  with  an  Haversian  canal  at  its  centre.  In  this  way 
the  bone  increases  in  diameter,  and  this  process  continues 
until  a  considerable  thickness  of  Haversian  system  bone  is 
formed.  In  all  bone  growth  there  is  the  alternation  of 
formation,  destruction,  and  rebuilding,  and  it  must  be 
remembered  that  this  continues  as  long  as  the  bone  functions 
as  an  organ  of  support.  As  the  shaft  becomes  larger  the 
primary  marrow  spaces  at  the  centre  are  enlarged  by  the 
absorption,  and  a  few  lamellae  are  laid  down  again  upon 
their  walls,  until  finally  in  the  central  portion  of  the  shaft 
the  true  marrow  cavity  is  formed.  As  -the  thickness  of 
Haversian  system  bone  becomes  greater,  absorptions  occur 


GROWTH  OF  MEMBRANE  BONES  261 

in  the  Haversian  canals,  cutting  out  large,  irregular  channels, 
around  which  a  few  lamellse  are  laid  down,  and  so  the  Haver- 
sian system  bone  becomes  converted  into  cancellous  bone  and 
is  opened  into  the  marrow  cavity  as  it  grows  larger. 

Growth  of  Membrane  Bones. — The  growth  of  the  membrane 
bone  progresses  in  a  very  similar  way.  As  soon  as  the 
periosteum  is  formed  subperiosteal  bone  is  laid  down  and 
converted  into  Haversian  system  bone,  forming  the  compact 
plate  of  the  surface,  leaving  the  cancellous  portion  first 
formed  at  the  centre.  When  a  certain  thickness  of  compact 
bone  has  been  formed,  absorptions  occur  in  the  Haversian 
canals,  converting  the  deeper  portions  into  cancellous  bone. 
This  process  may  be  reversed.  Absorptions  may  occur  under 
the  periosteum,  cutting  deeply  into  the  Haversian  system 
bone,  and  then  a  few  subperiosteal  layers  be  laid  down 
upon  it.  When  this  occurs  lamellae  are  laid  down  around 
the  marrow  spaces,  converting  the  cancellous  bone  into 
Haversian  system  bone  to  maintain  the  required  strength. 
In  this  way  the  bones  are  moulded  into  shape,  adapting 
them  to  the  mechanical  conditions  to  which  they  are  sub- 
jected. There  is  an  oscillation  between  formation  and 
destruction,  by  which  the  balance  adapted  to  the  mechanical 
conditions  is  maintained.  It  has  often  been  noted  that 
bones  are  never  allowed  to  become  more  bulky  than  is 
necessary  to  perform  their  function. 


CHAPTER  XX 

PERIOSTEUM^ 

Definition. — The  periosteum  is  the  formative  and  pro- 
tective membrane  which  covers  the  outer  surface  of  the  bone. 
All  periosteum  has  certain  structural  characteristics  in 
common,  but  because  of  structural  differences  two  classes 
are  recognized — attached  and  unattached — each  of  w^hich 
may  be  simple  or  complex.  Periosteum  may  thus  be  classi- 
fied as  follows: 

1.  Unattached  simple. 

2.  Unattached  complex. 

3.  iVttached  simple. 

4.  Attached  complex. 

Function  of  Periosteum. — The  importance  to  the  dentist  of 
a  knowledge  of  the  structure  and  function  of  the  periosteum 
can  scarcely  be  exaggerated.  It  has  been  the  knowledge  of 
this  tissue  and  its  function  that  has  led  to  all  the  advance- 
ment in  bone  surgery  of  modern  time.  Repair  and  regenera- 
tion of  bone  is  largely  accomplished  through  its  agency. 

The  periosteum  forms  the  immediate  covering  of  all  the 
bones  and  is  continuous  over  their  entire  surface  except  the 
portion  covered  by  cartilage.  Each  bone,  therefore,  has  a 
periosteum  of  its  own  which  does  not  continue  around  the 
articulation  to  the  bones  with  which  it  joins.  Bones  that 
are  united  by  suture  are,  however,  covered  by  a  common 
periosteum.    If  the  flesh  and  overlying  tissues  are  carefully 

1  In  the  presentation  of  this  chapter  it  is  impossible  adequately  to  express  my 
indebtedness  to  Dr.  G.  V.  Black.  Almost  all  of  the  illustrations  are  taken  from  The 
Periosteum  and  Peridental  Membrane,  published  by  him  in  1887.  I  have  always  felt 
that  this  book  had  never  received  the  attention  it  deserves.  Only  one  thousand  copies 
of  it  were  printed,  and  they  were  not  sold  until  the  orthodontists  exhausted  the  edition. 
The  book  is  now  entirely  out  of  print  and  is  very  difficult  to  obtain.  I  have  studied 
this  book  for  years  and  have  repeated  almost  all  of  the  work  described  in  it,  but  I  have 
felt  that  it  was  impossible  for  text-book  purposes  to  improve  upon  the  illustrations. 


FUNCTION  OF  PERIOSTEUM  263 

removed  from  a  long  bone,  the  periosteum  will  be  seen 
as.  a  smooth  white,  lustrous  membrane,  having  much  the 
appearance  of  a  tendon  on  most  of  its  surface.  But  at  some 
places  which  correspond  to  the  positions  where  muscles  or 
fascia  were  attached  it  appears  ragged  and  dull,  for  the 
tissues  had  to  be  cut  to  separate  them  from  the  outer  layer 
of  the  periosteum,  to  which  they  were  firmly  adherent.  In 
all  other  places  the  tissues  separate  easily  in  dissection;  in 
fact,  are  not  attached  at  ah,  except  by  the  lightest  of  areolar 
tissue,  which  is  very  easily  broken,  and  the  tissues  may  be 
separated  from  the  surface  of  the  membrane  with  the  finger 
or  the  handle  of  a  scalpel.  Now,  if  the  periosteum  is  slit 
along  a  smooth  surface  with  the  scalpel  and  the  handle 
inserted  between  the  bone  and  the  membrane,  it  will  be 
found  to  separate  readily  from  the  bone  over  most  of  its 
surface.  If  the  process  is  watched  closely,  little  strings  will 
be  seen  apparently  running  from  the  periosteum  to  the 
bone,  and  being  broken  as  they  are  separated.  These  are 
mostly  small  bloodvessels  which  are  running  into  canals  in 
the  bone.  In  this  process  the  periosteum  seems  like  a  closely 
adapted  sac  or  elastic  glove,  clothing  the  surface  of  the 
bone,  as  if  surrounding  it  in  a  fibrous  bag.  If  the  separation 
of  the  periosteum  from  the  bone  is  continued,  it  will  be  found 
that  it  does  not  separate  as  easily  in  all  places.  As  the 
articular  ends  are  approached  it  becomes  suddenly  fastened 
to  the  underlying  bone,  and  the  blade  of  the  knife  must  be 
used.  The  periosteum  now  appears  as  a  very  thin,  tough, 
and  inelastic  membrane,  that  is  torn  with  difficulty,  but  it  is 
so  thin  that  it  is  difficult  now  to  separate  it  from  the  bone 
without  cutting  it  through.  When  this  point  of  attachment 
is  reached  it  seems  that  the  periosteum  is  sinking  into  the 
substance  of  the  bone,  and  from  the  examination  of  its 
structure  it  is  found  that  this  is  practically  what  has 
happened. 

Comparing  the  periosteum  to  a  sac  surrounding  the 
bone,  it  is  found  sewed  firmly  down  at  the  margin  of  the 
cartilage  around  the  articular  ends.  Besides  the  attach- 
ment around  the  cartilage,  the  periosteum  will  be  found 


264  PERIOSTEUM 

adherent  in  the  following  positions:  Where  muscles  or 
fascia  are  attached  to  the  outer  layer  of  the  periosteum; 
where  it  approaches  the  insertion  of  tendons  or  ligaments; 
and  where  the  skin  or  mucous  membrane  seem  attached 
to  the  underlying  bone,  as  around  the  auditory  meatus, 
the  gums,  mucous  membrane  of  the  nose,  etc.  In  all  such 
positions  the  periosteum  is  firmly  attached  to  the  bone — in 
fact,  becomes  a  part  of  it — and  through  this  medium  the 
connections  between  muscles,  fascia,  etc.,  and  the  framework 
of  the  skeleton  is  accomplished. 

This  feature  of  the  anatomy  of  the  periosteum  has  never 
been  studied  in  the  detail  it  deserves,  especially  by  the 
dentist.  It  is  of  the  greatest  importance  in  the  manage- 
ment of  the  diseases  of  bone,  especially  those  involving 
the  formation  of  pus,  for  these  lines  of  attachment  deter- 
mine the  direction  in  which  the  pus  will  proceed  along  the 
surface  of  the  bone.  When  pus  generated  within  the  bone 
reaches  the  surface,  it  will  lift  an  unattached  periosteum 
and  run  along  the  surface  until  it  reaches  a  line  of  attach- 
ment. Here  it  can  penetrate  the  periosteum  more  easily 
than  it  can  separate  it  from  the  bone.  W^hen  a  line  of  attach- 
ment is  reached,  therefore,  the  direction  of  the  burrowing  is  de- 
termined by  the  attached  areas.  The  pus  penetrates  the  peri- 
osteum more  easily  than  it  separates  its  attachments  from 
the  bone,  but  it  lifts  the  unattached  periosteum  so  easily  that 
it  will  often  run  along  a  line  of  attachment  for  a  long  distance. 

These  factors  often  become  of  great  importance  in  deter- 
mining the  position  in  which  alveolar  abscesses  will  point. 
For  instance,  if  an  abscess  from  a  bicuspid  root,  or  the 
mesial  root  of  a  molar,  reaches  the  surface  of  the  bone 
above  the  attachment  of  the  buccinator,  it  cannot  pene- 
trate its  attachment  and  pass  downward  to  open  on  the 
gum,  but  may  run  out  over  the  surface  of  the  muscle  and 
open  on  the  cheek,  producing  the  crow's  foot  scar  so  often 
seen.  An  abscess  from  an  upper  cuspid  may  reach  the 
surface  of  the  bone  in  the  canine  fossa  between  the  attach- 
ments of  the  nasalis  and  canius,  and  lift  the  periosteum 
extending  upward,  and  open  at  the  inner  canthus  of  the  eye 


SIMPLE  UNATTACHED  PERIOSTEUM  265 

between  the  orbicularis  and  the  angular  head  of  the  quadratus 
labii  superioris.  If  these  abscesses  had  been  reached  with  a 
lance,  through  the  mucous  membrane,  at  the  proper  time, 
a  disfiguring  scar  would  have  been  avoided.  Accurate  knowl- 
edge of  the  attached  layers  of  the  periosteum  would  have 
made  it  certain  that  they  could  never  point  in  the  mouth 
cavity  without  assistance. 

Layers  of  the  Periosteum.— Periosteum  is  always  composed 
of  two  distinct  layers: 

1.  An  outer  or  fibrous  layer,  which  is  essentially  protective 
and  to  which  muscles  and  fascite  are  attached.  This  may  be 
either  simple  or  complex. 

2.  An  inner  or  osteogenetic  layer  which  is  essentially  the 
vital  functioning  layer,  and  is,  as  its  name  indicates,  con- 
cerned with  the  formation  of  bone.  This  may  be  either 
simple  or  complex. 

The  Structural  Elements. — The  periosteum  is  composed 
of  the  following  structural  elements: 

1.  White  fibers  in  coarse  bundles  (in  the  outer  layer). 

2.  White  fibers  in  very  fine  bundles  (in  the  inner  layer). 

3.  Elastic  fibers. 

4.  The  penetrating  fibers,  or  white  fibers  of  the  periosteum, 
that  in  the  growth  of  bone  are  included  in  its  substance. 

5.  Embryonal  connective-tissue  cells. 

6.  Osteoblasts  or  bone  forming  cells. 

7.  Osteoclasts  or  bone  absorbing  cells. 

Unattached  Periosteum. — In  the  unattached  periosteum  the 
inner  layer  is  always  simple,  and  the  outer  layer  may  be 
either  simple  or  complex,  depending  apparently  upon  the 
requirements  of  protection.  In  general,  the  more  exposed 
the  position  the  thicker  is  the  layer,  and  the  larger  and 
stronger  the  bundles  of  fibers  of  which  it  is  composed. 

Simple  Unattached  Periosteum. — Where  the  periosteum  is 
covered  by  a  thick  layer  of  muscles  which  are  not  attached 
to  it,  as  in  the  thigh,  the  thinnest  and  simplest  form  of 
periosteum  is  found.  An  illustration,  drawn  by  Dr.  Black, 
of  the  periosteum  from  the  femur  of  a  kitten  will  illustrate  its 
structure  (Fig.  208).    The  outer  layer  is  composed  chiefly  of 


266 


PERIOSTEUM 


bundles  of  white  fibers,  most  of  which  run  in  a  direction 
parallel  with  the  long  axis  of  the  bone.  The  bundles  are  com- 
paratively small  and  much  flattened,  so  as  to  be  quite  rib- 
bon-like. The  inner  layer  contains  a  much  greater  number 
of  cells  lying  among  extremely  delicate  fibers.  In  its  outer 
portion  many  of  the  cells  are  embryonal  in  character.  In 
contact  with  the  surface  of  the  bone  is  a  continuous  layer  of 


xl 


Fig.  208 


:K 


Non-attached  periosteum  from  the  shaft  of  the  femur  of  the  kitten:  B,  bone;  0, 
layer  of  osteoblasts.  In  the  central  portion  of  the  figure  they  have  been  pulled  slightly 
away  from  the  bone,  displa>-ing  the  processes  to  advantage.  It  will  be  observed  that 
the  fibers  of  the  periosteum  do  not  enter  the  bone,  a,  inner  layer  of  fine  white  fibrous 
tissue  (osteogenetic  layer)  showing  the  nuclei  of  the  fibroblasts  and  a  number  of 
developing  connective-tissue  cells,  which  probably  become  osteoblasts;  c,  outer  layer, 
or  coarse  fibrous  layer,  in  which  fusiform  fibroblasts  are  also  rendered  apparent 
by  double  staining  with  hematoxylin  and  carmine;  d,  some  remains  of  the  reticular 
tissue  connecting  the  superimposed  tissue  -with  the  periosteum.  (y_r  immersion.) 
(Black.) 

osteoblasts  which  are  building  subperiosteal  bone  in  the 
young  animal,  processes  of  their  cytoplasm  extending  into 
the  canaliculi  of  the  matrix  which  they  have  formed.  At 
one  point  in  the  illustration  the  osteoblasts  are  pulled  off 
from  the  surface  of  the  bone  and  show  these  processes 
stretched  out  of  the  canaliculi. 


ATTACHED  PERIOSTEUM  267 

Complex  Unattached  Periosteum. — In  some  places,  espe- 
cially where  muscles  or  tendons  perform  sliding  movements 
over  an  unattached  periosteum,  the  outer  layer,  instead  of 
being  simple,  may  be  very  complex.  This  is  illustrated  in 
Dr.  Black's  drawing  (Fig.  209),  from  the  periosteum  of  the 
tibia  of  a  young  pig.     In  this  instance  the  outer  layer  is 


^: 


Fig.  209 


Periosteum  from  the  shaft  of  the  tibia  of  the  pig.  lengthwise  section,  showing  the 
complex  arrangement  of  fibers  in  the  coarse  or  outer  fibrous  layer  that  sometimes 
occurs  under  muscles  that  perform  sliding  movements  upon  it:  B,  bone;  O,  layer  of 
osteoblasts.  The  tissue  has  been  pulled  shghtly  away  from  the  bone  in  mounting  the 
section,  and  part  of  the  osteoblasts  have  clung  to  the  bone,  some  have  clung  to  the 
tissues,  while  others  are  suspended  midway,  their  processes  clinging  to  each,  a  layer 
of  fine  fibers;  inner  or  osteogenetic  layer  of  the  periosteum;  6,  first  lamina  of  the  coarse 
or  outer  fibrous  layer,  the  fibers  of  which  are,  in  this  case,  circumferential,  exposing 
the  cut  ends.  It  will  be  observed  that  there  are  ten  lamina  in  the  make  up  of  the  outer 
layer,  the  lengthwise  and  circumferential  fibers  alternating.  The  ones  marked  / 
and  i  are  very  delicate  ribbon-like  forms,  which  have  shifted  from  their  normal  posi- 
tion in  the  mounting  of  the  section,  so  as  to  present  their  sides  to  view  instead  of 
their  ends,  thus  displaj-ing  their  structure  to  advantage.  The  illustration  shows  how 
readily  separable  these  lamina  are.     I,  reticular  tissue.     (jV  immersion.)      (Black.) 

composed  of  very  much  flattened  bundles  of  white  fibers, 
arranged  alternately  longitudinally  and  circularly.  Ten 
layers  may  be  counted  in  the  section.  The  inner  layer  is  of 
the  same  character  as  in  a  simple  specimen. 

Attached    Periosteum. — The    attached    periosteum    differs 
from  the  unattached  by  having  the  fibers  of  the  inner  layer 


268 


PERIOSTEUM 


arrano:ed  in  bundles,  around  which  the  bone  matrix  is 
deposited  hy  the  osteobhists,  embedding  them  in  the  sub- 
stance of  the  matrix  and  calcifying  them  with  it.  These 
fibers  constitute  the  penetrating  fibers.  They  were  first 
described  by  Sharpey,  and  have  been  called  Sharpey's 
fibers.  He,  however,  apparently  did  not  understand  their 
importance  or  manner  of  formation.  The  fibers  of  the 
inner  layer  are  built  into  the  substance  of  the  bone  in  this 
way  wherever  tissues  are  attached  to  the  outer  layer  of  the 
periosteum. 


^       '^■^       -  '  N  ,*• 


Simple  attached  periosteum:  a,  bone;  h,  osteoblasts;  c,  fibers  of  the  inner  layer; 
D,  bloodvessels  of  the  inner  layer;  E,  outer  layer;  F,  muscle  fibers  attached  to 
outer  layer.     (Black.) 

Simple  Attached  Periosteum.— Where  the  pull  of  tissues 
attached  to  the  outer  layer  of  the  periosteum  is  in  one 
direction,  the  fibers  of  the  inner  layer  are  inclined  in  the 
same  direction  (Figs.  210  and  211).    As  the  surface  of  the  bone 


Fig.  211 


A  ptiotomicrograph  of  an  attached  periosteum  similar  to  Fig.  210.      From  the  alveolai 
process  of  a  sheep.      (About  80  X) 


Fig.  212 


r'    a 


Attached  periosteum  from  beneath  the  attachment  of  the  muscles  of  the  lower 
lip  of  the  sheep:  .4.,  bone;  S,  osteoblasts,  with  the  fibers  emerging  from  the  bone 
between  them;  C,  inner  layer  with  fibers  decussating  and  joining  the  inner  side  of  the 
coarse  fibrous  layer  in  opposite  directions  (this  is  rather  an  unusual  form  of  this 
layer  of  the  periosteum);  Z),  coarse,  fibrous  layer;  E,  attachment  of  muscular  fibers. 
(Black.) 


270  PERIOSTEUM 

is  approached  the  fibers  are  gathered  into  strong  bundles  to 
be  inserted  in  the  bone,  the  osteoblasts  covering  the  surface 
of  the  bone  everywhere  between  the  fibers.  The  outer  and 
inner  layers  are  united  by  the  interlacing  of  their  fibers. 
At  the  junction  of  the  outer  and  the  inner  layers  many 
bloodvessels  are  seen. 

Complex  Attached  Periosteum. — Where  the  pull  upon  the 
outer  layer  is  in  many  directions,  the  fibers  of  the  inner  layer, 
after  emerging  from  the  bone,  break  up  into  smaller  bundles 
and  anastomose  in  all  directions,  arching  around  to  interlace 
with  the  fibers  of  the  outer  layer,  and  in  this  way  they  sustain 
force  in  all  directions  (Fig.  212).  This  is  illustrated  in  Dr. 
Black's  drawing  of  a  section  of  attached  periosteum  from 
beneath  the  attachment  of  the  muscles  of  the  lower  lip  of  a 
sheep. 


CHAPTER  XXI 

THE   ATTACHMENT   OF   THE    TEETH 

That  the  teeth  are  not  a  part  of  the  osseous  system,  but 
are  appendages  of  the  skin,  supported  in  man  by  a  special 
development  of  bone  forming  the  alveolar  ridges  of  the 
maxillary  bones,  is  as  well  established  as  any  fact  concerning 
human  dentition.  The  work  of  Oscar  Hertwig,  published 
in  1874,  established  very  clearly  the  homology  existing  be- 
tween the  teeth  and  the  dermal  or  placoid  scales  of  the  ganoid, 
silurioid,  and  dipnoan  fishes,  both  as  to  similarity  of  structure 
and  development. 

Much  has  been  written  descriptive  of  the  teeth  of  various 
animals,  their  modifications  of  form,  and  attachment  to 
adapt  them  to  modifications  of  function,  and  various  classi- 
fications of  the  means  of  attachment  have  been  made.  Of 
these,  perhaps  the  best  and  most  logical  is  given  by  Charles 
Tomes  in  his  Dental  Anatomy,  describing  four  forms  of 
attachment:  (1)  By  fibrous  membrane;  (2)  by  hinge-joint; 
(3)  by  ankylosis;  (4)  by  insertion  in  a  socket. 

These  various  forms  of  attachment  will  be  taken  up,  and, 
if  possible,  the  comparison  between  them  and  the  evolution 
of  the  more  complicated  forms  from  the  simpler  will  be 
shown.  The  study  must  begin  with  an  examination  of  the 
structure  and  attachment  of  the  placoid  scales  and  the 
simplest  form  of  tooth,  as  illustrated  in  the  shark. 

Structure  of  Dermal  Scales. — The  dermal  scales  are  composed 
of  a  conical  cap  of  calcified  tissue  developed  from  within  out- 
ward, by  an  epithelial  organ,  and  corresponding  in  structure 
to  the  enamel.  This  cap  is  supported  upon  a  conical  papilla 
of  calcified  tissue  formed  from  without  inward,  and  corre- 
sponding to  dentine.  In  the  outer  layer  the  arrangement  of 
the  fine  tubules  through  the  calcified  matrix  correspond  very 


272 


THE  ATTACHMENT  OF   THE   TEETH 


closely  to  human  dentine,  but  in  the  inner  portions  it  is  to  be 
understood  only  by  considering  the  formation  of  the  dentine 
as  progressing  irregularly  over  the  surface  of  the  pulp  and  so 
dividing  the  pulp  tissue  into  portions  enclosed  in  large  canals, 
from  which  the  fine  tubules  radiate.  The  base  of  this  partially 
calcified  papilla  has  a  calcified  connective  tissue  built  on  to  it 
by  the  derma  or  connective-tissue  layer  of  the  skin,  which  cor- 
responds to  cementum  forming  the  basal  plate,  spreading  out 


Fig.  213 


/M 


m 


Showing  additions  of   bone  of   attachment  to  the  bone  of   the   jaw.      (Tomes) 

more  or  less  in  the  connective-tissue  layer  of  the  skin,  and  into 
which  the  fibers  of  this  layer  are  built,  so  attaching  the  den- 
ticle or  dermal  scale  to  the  deep  layer  of  the  coreum.  This 
tissue  very  exactly  resembles  cementum.  It  is  formed  on 
the  dentine  as  the  cementum  of  a  human  tooth  is,  and  shows 
the  connective-tissue  fibers  embedded  in  it.  In  the  ganoids 
the  basal  plates  of  adjoining  scales  unite,  forming  the  armor 
plates  of  such  fish  as  the  sturgeon  and  gar-pike,  and  the 
dentical  remains  projecting  from  the  surface  of  the  plates. 

Attachment  by  Fibrous  Membrane. — In  the  simplest  teeth, 
as  of  the  shark  (Lamna  cornubica.  Fig.  3),  which  are  typical 


ATT  AC  HM  EXT  BY  HINGE  JOINT 


273 


dermal  scales,  there  is  an  exactly  similar  method  of  attach- 
ment, which  may  be  taken  as  the  simplest  and  most  rudimen- 
tary, or  attachment  in  a  fibrous  membrane.  That  is,  there  is 
no  development  or  modification  of  the  arch  of  the  jaw,  and 
the  teeth  have  no  direct  attachment  to  the  bone;  in  fact 
(Fig.  213),  the  jaws  themselves  are  chiefly  cartilage. 


Fig.  2U 


Attachment  by  hinge  joint.  Tooth  of  a  hake:  a,  vasodentine ;  h,  pulp;  c, 
elastic  hinge;  d,  buttress  to  receive/,  formed  out  of  bone  of  attachment;  e,  bone 
of  jaw;  /,  thickened  base  of   tooth;  g,  enamel  tip.      (Tomes.) 


Attachment  by  Hinge  Joint. — The  formation  of  the  hinge 
attachment  as  illustrated  in  many  of  the  fishes  (Fig.  214), 
may  be  understood  as  a  modification  of  the  attachment  in  a 
18 


274  THE  ATTACHMENT  OF   THE  TEETH 

fibrous  membrane  in  a  more  highly  specialized  creature. 
These  hinged  teeth  are  found  in  many  fishes  and  in  the  poison 
fangs  of  snakes.  The  jaws  are  calcified,  and  the  basal  plate 
or  cementum  may  be  considered  as  confined  to,  or  specially 
developed  on,  one  side  of  the  dentine  papilla,  which  is  also 
more  highly  developed,  especially  in  snakes.  This  cementum 
is  built  and  calcified  around  the  fibers  of  the  fibrous  tissue 
which  pass  directly  to  the  bone  of  the  jaw  at  that  point. 
This  bone  is  to  be  regarded  as  an  addition  to  the  jaw  specially 
developed  for  each  tooth.  Thus,  there  is  not  only  a  modifica- 
tion in  the  arrangement  of  the  cementum,  but  a  development 
of  bone  for  attachment  of  the  tooth.  The  bloodvessels  pass 
through  the  fibers  of  the  hinge  to  the  pulp,  and  are  not 
affected  by  the  motion  of  the  tooth  on  the  hinge;  in  fact, 
the  pulp  seems  to  be  attached  to  the  hinge.  There  are  many 
complications  of  this  method  of  attachment,  but  this  may 
be  taken  as  the  type  and  the  manner  of  its  modification  from 
the  rudimentary  conditions.  The  distinction,  in  this  form 
of  attachment,  from  the  dermal  scale  consists  in  a  modifica- 
tion of  the  arrangement  of  the  cementum  of  the  basal  plate 
and  a  development  of  bone  from  the  jaw  to  attach  fibers 
which  pass  directly  from  cementum  to  bone.  It  should 
also  be  said  that  there  are  developments  in  the  hinge  teeth 
related  to  the  third  form  of  attachment,  namely,  ankylosis, 
which  cannot  be  understood  until  this  form  is  studied. 

Attachment  by  Ankylosis. — The  third  form  of  attachment, 
ankylosis  (Fig.  215),  or  direct  calcified  union  with  the  bone  of 
the  jaw,  cannot  be  understood  \\dthout  a  careful  study  of  the 
nature  and  formation  of  the  dentine  in  these  rudimentary 
teeth.  It  is  evident,  from  a  study  of  the  dentine  of  the 
dermal  scales,  that  compared  with  human  dentine,  the  tissue 
is  rudimentary  and  not  differentiated  from  other  similar 
connective  tissues.  The  tubules  are  comparatively  very 
irregular,  and  resemble  strikingly  the  tubules  found  in  the 
secondary  dentine  formed  by  a  degenerating  pulp.  The 
odontoblasts,  or  dentine-forming  cells,  are  not  like  the  highly 
specialized  cells  which  form  the  primary  human  dentine, 
but  resemble  very  closely  simple  spindle-shaped  connective- 


ATTACHMENT  BY  ANKYLOSIS 


275 


tissue  cells.    The  nucleus  is  larger  and  oval  in  form,  and  the 
protoplasm  stretches  off  from  it  in  one  direction  intQ  a  fibril 


Fig.  21; 


Tooth  of  scarus,  showing  attachment  by  ankylosis.      (Owen.) 


276  THE  ATTACHMENT  OF   THE   TEETH 

instead  of  in  two  directions  into  a  spindle.  The  cells  are 
much  smaller  than  human  odontoblasts  and  nearer  the  size 
of  ordinary  spindle  cells  of  the  human  pulp.  In  fact,  they 
look  more  like  specially  developed  spindle  cells  than  odonto- 
blasts. The  formation  of  dentine  begins  on  the  surface,  at 
the  apex  of  a  cone-shaped  papilla  of  connective  tissue,  and 
proceeds  inward.  If  the  formation  continues  uniformly 
over  the  surface  of  the  papilla,  a  solid  layer  of  fine  tubuled 
dentine  results;  but  it  often  proceeds  irregularly,  apparently 
having  special  reference  to  the  neighborhood  of  bloodvessels, 
so  that  irregular  projections  of  dentine  are  found  on  its 
inner  surface,  dividing  the  pulp  more  or  less  into  portions 
enclosed  in  larger  channels  or  tubes.  These  may  be  very 
regular  in  arrangement  and  form  around  bloodvessels  loops 
embedding  the  bloodvessel  in  the  calcified  tissue,  producing 
what  has  been  called  vaso  or  vascular  dentine;  but  the 
formation  is  still  from  the  surface  of  the  pulp  until  it  is 
obliterated,  except  for  what  remains  in  the  larger  canals. 
As  distinguished  from  this  formation  of  dentine  we  find  in 
the  body  of  the  dental  papilla  of  many  fishes  the  formation 
of  spicules  of  calcified  tissue,  which  resemble  neither  den- 
tine nor  typical  bone,  shooting  down  through  the  substance 
of  the  pulp.  They  are  more  to  be  compared  with  the  first 
formation  of  bone  in  membranes,  or  in  the  embryonal 
connective  tissue  of  the  body  of  the  human  jaw,  which  is 
afterward  removed  by  absorption  and  replaced  by  true 
Haversian  system  bone.  These  calcifications  contain  lacunae, 
and  have  tubules  or  canaliculi  running  through  them,  and  so, 
as  Tomes  says,  are  intermediate  between  dentine  and  bone. 
They  divide  the  pulp  into  irregular  spaces,  and  interdigitate, 
or  perhaps  actually  join,  the  formation  of  dentine  which  has 
been  progressing  from  the  surface  of  the  pulp.  These 
spicules  run  down  into  the  bone  of  the  jaw,  forming  an 
actual  calcified  attachment  for  the  tooth  with  the  jaw;  but 
in  this  view  of  it  it  is  to  be  regarded  as  a  calcification  or 
rather  a  formation  of  bone  in  the  pulp  papilla  interlocking 
with  the  dentine.  In  some  of  the  fishes,  as  in  Scarus,  there 
is  at  the  same  time  the  remains  of  the  cementum  of  the  basal 


ATTACHMENT  BY  IMPLAXTATIOX  IX  SOCKET     277 

plate  formed  on  the  outside  of  the  dentine  around  the  base  of 
the  cone.  Ankylosis  is  confined  to  the  teeth  of  many  fishes, 
and  may  be  stated  as  a  modification  from  the  dermal  scale, 
resulting  in  the  reduction  or  loss  of  the  basal  plate  and  an 
ossification  of  the  pulp  continuing  through  the  connective 
tissue  at  the  base  of  the  pulp  to  the  body  of  the  jaw. 

Attachment  by  Implantation  in  Socket. — The  development 
of  the  fourth  form  of  attachment,  by  implantation  in  a 
socket,  seems  to  be  an  evolution  starting  from  the  same 
point  but  proceeding  in  a  different  direction  (Fig.  216).     It 


Fig.  216 


.-i,  diagrams  of  transverse  sections  through  the  jaws  of  reptiles  showing  pleurc- 
dont  (a),  acrodont  (6),  and  theodont  (c)  dentitions.  B.  a,  lower  jaw  of  Zootoca 
vivipara;  b,  of  anguis  fragilis.  (After  Leydig  )  (Weidersheim,  Comparative 
Anatomy  of  Vertebrates.) 


is  associated  with  the  very  great  increase  in  the  size  of  the 
teeth  and  consequent  necessity  for  a  stronger  attachment. 
The  evolution  of  this  is  illustrated  in  the  teeth  of  reptiles. 
Weidersheim  classifies  the  teeth  of  reptiles  as  (1)  resting 
upon  a  ledge  on  the  lingual  side  of  the  jaw — pleurodont 
dentition;  (2)  resting  on  a  slight  ridge  around  them — acro- 
dont dentition;  (3)  lodged  in  permanent  alveoli,  as  in  the 
crocodile — theodont  dentiton.  These  three  classes  illus- 
trate three  stages  in  the  development  of  the  socket  method 
of  attachment. 

In  the  simplest  form  there  is  a  cone-shaped  tooth,  attached 


27S  THE  ATTACHMENT  OF  THE  TEETH 

to  the  bone  around  its  base,  by  the  fibers  being  built  into 
the  cementum  and  bone.  There  is  little  modification  of  the 
rudimentary  form,  and  little  development  of  bone  for  its 
attachment.  In  a  higher  form  the  tooth  has  become  long  or 
peg-shaped,  and  the  bone  has  grown  up  around  a  portion  of 
it  to  support  it;  but  it  is  attached  to  the  bone  by  connective- 
tissue  fibers,  being  built  into  the  cementum  on  the  surface 
of  the  tooth  and  into  the  bone  of  attachment  on  the  jaw. 
The  development  of  the  form  of  the  tooth  to  the  peg  from 
the  cone  may  be  understood  as  a  continuing  of  the  develop- 
ment of  odontoblasts,  and  the  formation  of  dentine  (which 
always  begins  at  the  apex  of  the  cone)  farther  and  farther 
down  the  sides  of  the  dental  papillae.  Then  the  formation 
of  the  cementum,  which  begins  around  the  base  of  the  cone 
and  continues  down  on  the  outside  of  the  calcified  dentine, 
covering  its  outer  surface,  and  building  the  connective-tissue 
fibers  into  the  tooth.  The  development  of  bone  accom- 
panies, or  rather  follows  that  of  the  tooth,  building  the 
other  ends  of  these  fibers  into  the  bone  which  is  developed 
to  support  the  tooth. 

Summary. — To  review  the  subject  matter  of  this  chapter, 
all  teeth  have  been  evolved  from  the  simple  placoid  scale. 
In  the  simplest  forms,  as  in  the  teeth  of  the  shark,  there  is 
no  relation  to  the  bone  whatever,  but  the  fibers  of  the  sub- 
cutaneous tissue  are  built  into  the  basil  plate  of  cementum. 
As  the  tooth  becomes  larger  and  demands  more  support, 
there  is  added  to  the  bone  of  the  jaw  that  which  Tomes  has 
called  bone  of  attachment.  The  osteoblasts  build  up  addi- 
tions to  the  jaw  which  surround  and  embed  the  fibers,  so  that 
the  fibers  which  were  originally  in  the  subcutaneous  tissue 
are  fastened  to  the  bone  at  one  end  and  to  the  cementum 
at  the  other.  The  evolutions  of  attachment  by  hinge  joint 
and  by  gomphosis  are,  therefore,  direct  evolutions  from  the 
simple  attachment  in  membrane.  The  form  of  ankylosis  is 
also  evolved  from  the  simplest  type,  but  in  this  case  the  bone 
of  attachment  is  associated  with  the  pulp,  and  the  formation 
of  bone  and  dentine  become  interlocked  and  united. 


CHAPTER  XXII 

THE  PERIDENTAL  MEMBRANE 

Ix  one  sense  the  peridental  membrane  may  be  consid- 
ered as  the  most  important  of  the  dental  tissues,  for  upon 
it  the  usefulness  of  the  teeth  and  their  comfort  to  the  indi- 
vidual is  dependent.  It  makes  no  difference  how  perfect  a 
crown  may  be,  or  how  perfectly  any  damage  which  may 
have  occurred  to  it  may  have  been  restored,  unless  the 
peridental  membrane  is  in  a  healthy  and  fairly  normal  con- 
dition, the  tooth  will  be  useless,  and  the  individual  would 
be  much  more  comfortable  without  it. 

Definition. — The  peridental  membrane  may  be  defined  as 
that  tissue  which  fills  the  space  between  the  surface  of  the 
root  and  the  bony  wall  of  its  alveolus,  surrounds  the  root 
occlusally  from  the  border  of  the  alveolus,  and  supports  the 
gingivus.  It  is  necessary  to  emphasize  the  three  parts  of 
the  definition.  The  peridental  membrane  does  not  stop 
at  the  border  of  the  bone,  but  continues  to  surround  the  root 
as  far  as  the  tissues  are  attached  to  it.  In  general,  the  dental 
profession  has  thought  of  the  peridental  membrane  as  only 
that  tissue  which  occupies  the  space  between  the  root  and  the 
wall  of  its  alveolus.  As  will  be  seen  from  a  study  of  a  section 
later  (Figs.  219  and  220),  the  structure  of  the  tissue  surround- 
ing the  root  between  the  gingival  line  and  the  border  of  the 
process  is  essentially  the  same  as  that  in  the  alveolus,  and 
quite  difterent  from  the  much  coarser  fibrous  mat  forming 
the  submucous  layer  of  the  gum  tissue.  The  peridental 
membrane  also  extends  into  the  free  margin  of  the  gum  and 
is  the  means  of  its  support,  holding  the  gingivae  close  to  the 
surface  of  the  tooth  and  supporting  them  in  the  interproximal 
spaces.     The  importance  of  this  portion  of  the  peridental 


280  THE  PERIDENTAL  MEMBRANE 

membrane  and  the  functions  which  it  performs  have  been 
strongly  emphasized  in  the  last  few  years,  in  their  relation 
to  the  extensions  of  caries  and  the  beginnings  of  pyorrhea. 
Most  of  the  diseases  of  the  peridental  membrane  which 
result  in  the  final  loss  of  the  teeth  have  their  beginnings  in 
this  portion. 

Nomenclature.^ — The  peridental  membrane  belongs  to 
the  class  of  fibrous  membranes  which  form  the  covering 
of  organs,  the  capsules  of  glands,  and  especially  those 
membranes  which  cover  the  organs  of  support.  Its  closest 
relative  is  the  periosteum  in  the  attached  portions,  with 
which  it  has  many  points  of  structure  in  common,  but 
it  differs  from  the  periosteum  in  any  position  in  important 
respects.  It  has  often  been  called  the  alveodental  peri- 
osteum, but  this  name  implies  that  the  periosteum  is  folded 
down  into  the  alveolus  and  back  upon  the  surface  of  the 
root,  which  is  an  entirely  erroneous  conception  of  the  mem- 
brane. This  idea  would  imply  that  it  was  a  double  mem- 
brane having  one  layer  covering  the  bone  and  another 
covering  the  root,  the  two  uniting  in  the  middle  portions. 
But  instead,  the  periosteum  must  be  considered  as  stopping 
at  the  border  of  the  alveolus,^  and  being  united  with  the 
peridental  membrane  around  its  circumference.  ]\Iany 
writers  use  the  word  pericementum  in  place  of  peridental 
membrane.  The  author  prefers  and  in  this  book  will  use 
the  term  peridental  membrane,  though  the  two  are  synon}'- 
mous. 

Divisions. — Purely  for  convenience  in  description,  the 
peridental  membrane  is  divided  into  three  portions:  The 
gingival  portion,  that  portion  of  the  membrane  which  sur- 
rounds the  root  occlu sally  from  the  border  of  the  alveolar 
process  and  supports  the  gingivae;  the  alveolar  ])ortion,  the 
portion  of  the  membrane  from  the  border  of  the  process  to 
the  region  of  the  apex  of  the  root;  and  the  apical  portion, 

'  The  student  must  be  reminded  that  the  word  alveolus  means  a  hole,  and  the 
alveolar  process,  the  portion  of  the  bone  which  contains  the  holes.  In  dental  writing 
the  word  alveolus  has  often  been  incorrectly  used  in  place  of  process  or  alveolar 
process. 


PLATE   XI 


% 


V, 


V. 


4' 


"^s 


-■^.■. 


■  'f>  , 


i 


4. 


i'  ■■ 


^i?**l*^iwirff 


Longitudinal  Section  of  Peridental  Membrane. 

Stained  with   heniatoxylin  and  eosin.    Showing   border  of  alveolar  process. 


PLATE   XI  [ 


n:,i 


^  '^ 


f       • 


-ui 


\ 


i 


f     J   ,:        t  ^." 


\       '-S 


ii'^-'li^tfi.wia.  -v.. 


te  v§m 


Longitudinal  Section  of  Peridental  Membrane. 

Stained  with  hematoxylin  and  eosin.     Showing  part  of  the  Ungual  gingivus 
and   border  of  the  alveolar  process. 


PLATE   XIII 


% 


J  7 
-J 


m 


S-      4'.   - 


/ 


.'-*'* 


ii4 


H 


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/ 


Transverse    Section    of   Peridental    Membrane. 

Stained  with  liematoxylin  and  eosin.     Alveolar  portion. 


FUNCTIONS 


281 


which  surrounds  the  apex  of  the  root  and  fills  the  apical 
space.  These  are  illustrated  in  the  diagram  (Figs.  217  and 
218). 

The  Structural  Elements. — These  are:  (1)  White  connective- 
tissue  fibers;  (2)  fibroblasts;  (3)  cementoblasts;  (4)  osteo- 
blasts; (5)  osteoclasts;  (6)  epithelial  structures  which  have 
sometimes  been  called  the  glands  of  the  peridental  membrane; 
(7)  bloodvessels;  and  (8)  nerves. 

Fig.  217 


Drawing  to   show    the  arrangement  of   the   fibers  in   a  labiolingiial   section  through 
an  incisor  of   a  kitten.      (Black  ) 


Functions. — ^The  peridental  membrane  performs  three 
functions:  (1)  A  ph^-sical  function — it  maintains  the  tooth 
in  relation  to  the  adjacent  hard  and  soft  tissues.  (2)  A 
vital  function — the  formation  of  bone  on  the  alveolar  wall 
and  of  cementum  on  the  surface  of  the  root.  (3)  A  sensory 
function — the  sensation  of  touch  for  the  tooth  being  exclu- 
sively in  this  membrane. 

It  is  necessary  to  emphasize  the  two  parts  of  the  physical 


282 


THE  PERIDENTAL  MEMBRANE 

Fig.  218 


G    -! 


Al 


Ap  •! 


Diagram  of  the  fibers  of  the  peridental  membrane:  G,  gingival  portion;  Al, 
alveolar  portion;  Ap,  apical  portion.  (From  a  photograph  of  a  section  from  incisor 
of  sheep.) 


ARRANGEMENT  283 

function;  the  peridental  membrane  not  only  supports  the 
teeth  in  their  relation  to  the  bones  which  carry  them,  and 
sustains  them  against  the  forces  of  occlusion  and  masti- 
cation, but  it  also  sustains  the  soft  tissues  in  their  proper 
relation  to  the  teeth.  The  second  part  of  the  physical 
function  is  fully  as  important  as  the  first,  and  the  study  of 
the  structure  of  the  tissue  related  to  it  and  the  adaptation  of 
the  form  of  the  gingivse  to  the  anatomic  form  of  the  teeth 
and  alveolar  process,  are  important  considerations  which 
should  never  be  lost  sight  of  in  the  making  of  artificial 
crowns. 

Classes  of  Fibrous  Tissue. — The  fibrous  tissue  of  the 
peridental  membrane  is  entirely'  of  the  white  variety,  but 
may  be  divided  into  two  classes.  The  principal  fibers  and 
the  indifferent  or  interstitial  tissue.  The  former  perform  the 
physical  function  of  the  membrane,  the  latter  simply  fill  in 
spaces  between  the  bundles  of  fibers  and  surround  and 
accompany  the  bloodvessels  and  the  nerves. 

The  Principal  Fibers  of  the  Peridental  Membrane. — These 
may  be  defined  as  the  fibers  which,  springing  from  the 
cementum,  are  attached  at  their  other  extremities  to  the 
connective  tissue  supporting  the  epithelium,  the  fibrous  mat 
of  the  gum  tissue,  the  cementum  of  the  approximating  tooth, 
the  outer  layer  of  the  periosteum  at  the  border  of  the  alveolar 
process,  or  the  bone  of  the  alveolar  wall. 

Arrangement.— The  principal  fibers  literally  spring  from 
the  cementum,  the  cementoblasts  building  up  the  matrix 
around  them  and  then  calcifying  both  the  matrix  and  the 
fibers,  in  this  way  attaching  them  to  the  surface  of  the  root. 
In  most  places  the  fibers  as  they  spring  from  the  cementum 
appear  as  good-sized  bundles.  A  short  distance  from  the 
surface  of  the  root  they  may  break  up  into  smaller  bundles 
which  anastomose  and  interlace,  passing  around  bloodvessels 
and  other  fibers  in  their  course  and  being  again  united  into 
large  bundles  for  attachment  at  their  other  extremity. 

To  arrive  at  an  understanding  of  the  arrangement  of  the 
fibers  of  the  peridental  membrane,  sections  must  be  cut 
longitudinally,  both  from  buccal  to  lingual  and  from  mesial 


2S4  TIIE  PKRJJJKXTAL  MEMBRANE 

to  distal,  and  transversely  through  all  portions  of  the  mem- 
brane. It  therefore  re(iuires  the  study  of  many  sections  to 
work  out  a  complete  conception.  After  studying  them  out 
completely  in  this  way  one  is  impressed  with  the  beautiful 
adaptation  of  their  arrangement  to  sustain  the  tooth  against 
all  the  forces  to  which  it  is  subjected,  and  to  support  the  free 
margin  of  the  gum,  so  that  it  will  lie  closely  against  the 
gingival  portion  of  the  enamel.  It  is  necessary,  however, 
to  remind  the  student  that  connective  tissues  are  formed  in 
response  to  mechanical  conditions  and  stimuli,  and  therefore 
this  arrangement  must  be  considered,  not  as  having  been 
designed  to  sustain  the  forces,  but  as  being  the  result  of  the 
forces  to  be  sustained,  and  therefore  beautifully  adapted  to 
them. 

Beginning  at  the  gingival  line,  the  fibers  springing  from 
the  cementum  pass  out  at  a  short  distance  at  right  angles 
to  its  surface  and  then  bend  sharply  to  the  occlusal, 
passing  up  into  the  gingivus  and  uniting  with  the  fibrous 
mat  which  supports  the  epithelium.  These  are  much  more 
strongly  marked  on  the  Ungual  than  on  the  labial  gingivus, 
because  in  mastication  the  lingual  gingivus  receives  more 
pressure  of  food,  which  would  tend  to  crush  it  down.  A 
little  deeper  the  fibers  springing  from  the  cementum  on  the 
labial  and  lingual  pass  out  at  right  angles  to  the  cementum 
and  are  lost  in  the  coarser  fibrous-mat  of  the  gum  tissue. 
The  distance  which  they  extend  before  lost  in  the  coarser 
fibers  is  always  greater  on  the  lingual  than  on  the  labial. 
On  the  proximal  sides  the  fibers  springing  from  the  cementum 
at  the  same  level,  branch  and  interlace,  passing  across  the 
interproximal  space,  to  be  attached  to  the  cementum  of 
the  approximating  tooth.  These  fibers  are  of  the  greatest 
importance,  as  they  produce  the  basket  work  which  forms 
the  supporting  framework  for  the  interproximal  gingivus. 
A  little  farther  occlusally  the  fibers  as  they  come  from  the 
cementum  are  inclined  apically.  A  short  distance  from  the 
cementum  they  unite  into  very  large  and  strong  bundles 
which  join  with  the  fibers  of  the  outer  layer  of  the  perios- 
teum, extending  over  the  labial  and  lingual  border  of  the 


ARRAXGEMEXT  285 

alveolar  process.  On  the  proximal  sides  the  fibers  at  the 
same  level  are  attached  to  the  cementum  of  the  adjoining 
tooth,  or  are  inclined  apically,  to  be  inserted  in  the  bone  of 
the  septum.  These  large  bundles  form  a  distinct  la^'er, 
which  has  been  called  the  dental  ligament,  and  bind  them 
together  across  the  septum.  They  are  the  only  fibers 
which  hold  the  teeth  down  in  its  socket.  At  the  border 
of  the  alveolar  process,  and  in  the  occlusal  third  of  the 
alveolar  portion,  the  fibers  pass  directly  from  the  cementum 
to  the  bone  at  right  angles  to  the  axis  of  the  tooth.  In 
this  position  the  fibers  are  larger  and  stronger,  and  show 
less  tendency  to  break  up  into  smaller  bundles  in  their 
course  than  in  any  other  portion  of  the  membrane.  In  the 
middle  and  apical  thirds  of  the  alveolar  portion  the  fibers 
are  inclined  occlusally  as  they  pass  from  the  cementum 
to  the  bone.  They  spring  from  the  cementum  in  compact 
bundles,  and  show  a  strong  tendency  to  break  up  into 
fan-shaped  fasciculi,  spreading  out  as  they  approach  the 
bone,  to  be  attached  over  a  larger  area  of  the  alveolar  wall. 
These  fibers  literally  swing  the  tooth  in  its  socket  and 
support  it  against  the  forces  of  mastication.  In  the  apical 
region  fibers  springing  from  the  cementum  pass  out  in  all 
directions,  spreading  out  in  the  same  way,  to  be  inserted 
into  the  bone  forming  the  wall  of  the  apical  space. 

If  force  is  exer^d  against  the  lingual  surface  of  an  incisor, 
the  fibers  on  the  lingual  side  of  the  root  in  the  occlusal  third 
will  sustain  part  of  the  strain,  preventing  the  cro\NTi  from 
moving  labially,  and  at  the  same  time  the  fibers  on  the  labial 
side  of  the  root  in  the  apical  space  will  also  be  under  strain, 
preventing  the  apex  of  the  root  from  moving  lingually. 
The  general  plan  of  arrangemenit  which  has  been  described 
is  illustrated  in  Dr.  Black's  diagram  made  from  a  labio- 
lingual  section  of  an  incisor  of  a  young  kitten  (Fig.  217). 

With  this  general  plane  of  arrangement  in  mind  individual 
sections  may  be  studied,  examining  the  arrangement  and 
appearance  of  the  fibers  in  detail.  Figs.  219  and  220  show 
the  labial  and  lingual  gingivae  from  an  incisor  of  a  sheep. 
Notice  that  the  labial  gingivus  is  taller  and  thinner,  and  the 


286 


THE  PERIDENTAL  MEMBRANE 


fibers  passing  up  into  it  are  not  as  strongly  marked.  Notice 
also  the  distance  to  which  the  final  fibers  of  the  peridental 
membrane  can  be  followed  before  they  are  lost  in  the  coarser 
mat  of  gum  tissue.  The  lingual  gingivus  is  broader  and 
flatter,  and   the  fibers   passing   up   into   it   form  a  strong 

Fia.  219 


•\^^ 


Longitudinal  section  of  the  peridental  membrane  in  the  gingival  portion, 
from  a  lamb  (the  labial  gingivus). 

and  well-defined  band.  Under  higher  magnification,  fibers 
would  be  seen  cut  transversely  in  the  gingivus,  which 
pass  around  the  tooth,  helping  to  hold  it  closely  against 
the  enamel.     In  Fig.  221  the  fibers  uniting  with  the  outer 


ARRAXGEMENT 


287 


layers  of  the  periosteum  are  very  well  shown.  Taking 
transverse  sections  in  the  gingival  portion  and  remem- 
bering that  they  are  cut  at  right  angles  to  these  through 
the  same  area,  the  distribution  of  the  tissues  will  be  bet- 
ter understood.    Fig.  222  shows  a  section  cut  close  to  the 

Fig.  220 


Longitudinal  section  of  the  peridental  membrane  in  the  gingival  portion  (the 
lingual  gingivus):  D,  dentine;  N,  Nasmyth's  membrane;  C,  cementum;  F,  fibers  sup- 
porting the  gingivus;  Fi,  fibers  attached  to  the  outer  layer  of  the  periosteum  over 
the  alveolar  process;  F-,  fibers  attached  to  the  bone  at  the  rim  of  the  alveolus; 
B,  bone.     (About  30  X) 


gingival  line.  At  A  the  epithelium  on  the  labial  surface  of 
the  gingivus  is  seen,  and  at  B  the  epithelium  lining  the 
gingival  space.  On  the  proximal  sides  of  the  roots  the  fibers 
will  be  seen  passing  from  the  cementum  of  one  tooth  to  that 
of  the  next.    Fig.  223  is  a  httle  deeper  and  shows  the  fibers 


Fig.  221 


Longitudinal  section  of  peridental  membrane  of  young  sheep,  showing  fibers 
penetrating  the  cementum:  D,  dentine;  C,  cementum,  showing  embedded  fibers; 
F,  fibers  running  to  the  outer  layer  of  the  periosteum,  covering  the  aveolaf  process; 
F',  fibers  running  to  the  bone  at  the  border  of  the  process;  B,  bone.     (About  80  X) 


ARRANGEMENT 


289 


attached  around  the  entire  circumference  of  the  root.  Begin- 
ning at  the  middle  of  the  labial  surface,  the  fibers  will  be 
found  springing  from  the  cementum  and  passing  out  at 
right  angles  to  it,  to  be  lost  in  the  fibrous  mat  supporting 
the  epithelium.  The  fine  fibers  of  the  peridental  membrane 
can  be  followed  for  about  half  the  distance  to  the  epithelium 
before  they  are  lost  in  the  coarser  mat  of  gum  tissue,  and 
a  fairlv  definite  boundary  will  be  seen  between  what  should 


Transverse  section  of  the  peridental  membrane  in  the  gingival  portion,  from 
young  sheep.  The  roots  of  two  temporarj'  incisors  are  cut  across.  The  epitheUum 
lining  the  gingival  space  is  shown  part  way  around  one.  A,  epitheUum  on  labial 
surface  of  gingivae;   B,  epithelium  lining  the  gingival  space.      (About  60  X) 


be  considered  peridental  membrane  and  the  gum  tissue.  As 
the  distolabial  angle  of  the  root  is  approached,  the  fibers 
passing  from  the  cementum  ten.d  to  swing  around  distally, 
and  pass  to  the  mesiolabial  angle  of  the  adjoining  tooth. 
Along  the  proximal  surface  the  network  which  supports  the 
interproximal  gingivus  is  well  shown.  The  fibers  springing 
from  the  cementum  interlace  and  pass  around  bloodvessels 
and  fibers  which  are  passing  up  into  the  gingivus,  and  finally 
19 


290 


THE  PERIDENTAL  MEMBRANE 


are  inserted  into  the  cementiim  of  the  next  tooth.  In  this 
way  it  will  l^e  seen  that  the  teeth  in  the  entire  arch  are  firmly 
bound  together  by  the  fibers  in  the  gingival  portion.    This 


Fig.  223 


Transverse  section  of  the  peridental  membrane  in  the  gingival  portion  (from 
sheep):  E,  epithelium;  F,  fibrous  tissue  of  gum;  B,  point  where  peridental  mem- 
brane fibers  are  lost  in  fibrous  mat  of  the  gum;  P,  pulp;  F' ,  fibers  extending  from 
tooth  to  tooth.      (About  30  X  ) 


explains  the  way  in  which  the  positions  of  all  the  teeth  are 
affected  by  the  loss  of  a  single  one  in  the  arch,  and  the  way 
in  which  the  movement  of  one  tooth  will  draw  its  neighbors 
after  it.     It  also  explains  the  separation  of  the  central 


PLATE   XiV 


Transverse     Section     of    the     Peridental     Membrane    in     the 
Occlusal  Third  of  the  Alveolar  Portion  (from  Sheep). 

.V,  muscle  fibers;    Per,  periosteum;    .4/,  bone  of  the  alveolar  process;   Pd,  peri- 
dei-ital   nien-ibrane   fibers;   P,  pulp;   D.  dentine;   Cm,  eementum. 


ARRANGEMENT 


291 


incisors  when  the  freniim  labium  passes  through  between  the 
teeth,  and  is  inserted  on  the  lingual  surface  of  the  alveolar 
process.  If  these  incisors  are  to  be  held  together  perma- 
nently, normal  attachment  of  fibers  extending  from  the 
cementum  of  one  tooth  to  that  of  the  other  must  be  secured. 
The  fibers  in  this  area  are  also  well  shown  in  Fig.  224,  and 


Fia.  224 


■'/-'. 
**  s 


A  portion  of  the  peridental  membrane  between  two  incisors  of  a  young  sheep, 
showing  the  fibers  extending  from  tooth  to  tooth. 


it  can  be  understood  how  they  form  foundation  upon  which 
the  interproximal  gingivus  rests.  The  first  step  in  the 
sagging  of  the  interproximal  gum  tissue  is  the  cutting  off 
of  the  fibers  from  the  cementum,  where  it  bends  occlusally, 
following  the  curve  of  the  gingival  line  on  the  proximal 
surface. 

Plate  XIV  shows  a  transverse  section  in  the  occlusal  third 


292 


THE  PERIDENTAL  MEMBRANE 

FiQ.  225 


■«^^-    y.-'.j  i      -uv 

W^ 

^^^!^fA  m^^ . 

0% 

^^4^^MP^^^K^^^9|H^^^H^^^jN^    t 

%                '-CV" 

»""* 

3  >« 

< 

Diagram  of  peridental   membrane  from  section  similar  to  Fig.   224.     (From  Maloc- 
clusion  of  the  Teeth,    Dr.  E.  H.  Angle.) 

Fig.   22G 


Fibers  at  the  border  of  the  alveolar  process  (from  sheep):  D,  dentine;  C, 
cementum;  F,  fibers  extending  from  cementum  to  bone;  Bl,  bloodvessel;  B,  bone. 
(About  80  X) 


A  RRA  N  GEM  EN  T  293 

of  the  alveolar  portion  from  the  incisor  of  a  sheep.  Upon 
the  labial  a  few  muscle  fibers  are  seen  and  the  periosteum 
covering  the  labial  surface  of  the  process.  Notice  the  medul- 
lary spaces  in  the  bone  and  the  canals  opening  into  the 
peridental  membrane  and  periosteum.  The  light  line  forming 
the  outer  boundary  of  the  dentine  is  characteristic.  Two 
layers  of  cement um  are  seen,  and  notice  the  thickening  of  the 
layer  where  strong  bundles  are  attached.  At  the  middle  of 
the  labial  surface  the  fibers  pass  at  right  angles  to  the  cemen- 
tum  and  are  attached  to  the  bone,  but  as  the  distolabial 
angle  of  the  root  is  approached  the  bundles  swing  distally  to 
be  attached  in  the  bone.  In  Fig.  225,  which  was  drawn  very 
carefully  from  this  section,  the  arrangement  of  the  fibers  is 
shown  diagrammatically.  Notice  the  way  in  which  they 
pass  over  and  under  each  other  and  around  the  bloodvessels 
which  wind  through  them.  This  relation  to  the  bloodvessels 
is  important,  and  will  be  considered  again  later  in  connection 
with  the  blood  supply  of  the  membrane.  The  tangential 
fibers  at  the  angle  of  the  root  hold  the  tooth  against  the 
forces  which  tend  to  rotate  it  in  its  socket.  They  are  impor- 
tant in  connection  with  all  rotating  movements  in  ortho- 
dontia. It  has  long  been  noted  that  rotations  were  the 
hardest  movements  to  retain,  especially  if  the  tooth  were 
moved  in  no  other  direction.  In  this  case,  if  the  tooth  were 
turned  mesially  the  fibers  at  the  distolabial  angle  would 
spring  the  thin  plate  of  the  alveolar  process  as  a  bow  is 
bent,  leaving  a  condition  of  stress  in  the  tissue  which  will 
tend  to  spring  back  into  its  old  position  and  drag  the  tooth 
with  it.  Notice  the  greater  thickness  of  the  membrane 
on  the  lingual  as  compared  with  the  labial.  Figs.  221  and 
226  show  longitudinal  sections  at  the  border  of  the  alveolar 
process.  Notice  that  the  fibers  can  be  seen  running  through 
the  entire  thickness  of  the  cementum.  They  are  large, 
strong  fibers  and  branch  \ery  little  in  their  course.  Note  the 
bloodvessel  that  is  shown  in  several  of  these  sections,  and 
the  way  in  which  it  gives  oft'  branches  passing  over  the  border 
of  the  processes  and  toward  the  cementum. 


CHAPTER  XXIII 

THE   CELLULAR   ELEMENTS   OF   THE   PERIDENTAL 
MEMBRANE 

Fibroblasts. — The  fibroblasts  are  found  everywhere  between 
the  fibers  which  they  have  formed  and  to  which  they  belong. 
They  are  spindle-shaped  or  stellate  connective-tissue  cells, 
having  a  more  or  less  flattened  nucleus  and  a  body  of  granu- 

Fig.  227 


Fibers  and   fibroblasts   from   transverse   section  of   membrane:   F,  fibers  cut  trans- 
versely;  F^,  fibers  cut  longitudinally,  showing  fibroblasts.      (About  80  X) 


lar  cytoplasm,  which  is  squeezed  out  into  thin  projections 
between  the  fibers.  In  sections  stained  with  hematoxylin 
the  cells  take  the  stain  strongly  and  the  fibers  remain  clear 
(Fig.  227).  In  this  way  the  fibers  are  marked  out  by  the 
cells  which  lie  between  them.  The  number  of  the  fibroblasts 


CEMEXTOBLASTS  295 

in  the  membrane  decreases  with  age.  They  are  large  and 
numerous  in  the  membrane  of  a  newly  erupted  tooth  and 
are  comparatively  small  and  few  in  the  membrane  around 
an  old  tooth.  This  is,  however,  characteristic  of  fibroblasts 
in  connective  tissue  generally.  Fig.  227  shows  a  small  field 
taken  from  the  gingival  portion  of  the  membrane  between 
the  teeth.  The  magnification  is  low,  the  photograph  being 
made  with  a  f  objective.  The  cells  are  seen  as  little  dark 
dots  lying  between  the  fibers,  which  are  clear.  Where  the 
fibers  are  cut  longitudinally  they  appear  spindle-shaped, 
but  where  the  fibers  are  cut  across  they  appear  star-shaped. 
They  will  be  seen  better  in  photographs  made  with  higher 
magnification,  but  an  adequate  idea  of  their  form  can  only 
be  obtained  by  studying  sections  very  carefully  with  a  J  or  -jV 
objective  and  using  the  fine  adjustment  to  gain  an  idea  of 
the  third  dimension  of  space.  They  are  shown  in  many  of 
the  illustrations  of  the  epithelial  structures. 

Cementoblasts. — The  cementoblasts  are  the  cells  which 
form  cementum.  They  cover  the  surface  of  the  root  every- 
where between  the  fibers  which  are  embedded  in  the  tissue. 
While  these  cells  perform  the  same  function  for  the  cementum 
as  the  osteoblasts  do  for  bone,  they  are  quite  dift'erent  in 
form.  They  are  always  flattened  cells,  sometimes  almost 
scale-like,  and  when  seen  from  above  very  irregular  in  out- 
line. This  irregularity  in  outline  is  due  to  the  projections  of 
the  cytoplasm  around  the  fibers  as  they  spring  from  the 
cementum,  the  edges  of  the  cell  being  notched  and  scalloped 
to  fit  about  them.  There  is  a  central  mass  of  granular 
cytoplasm  which  contains  an  oval  and  more  or  less  flattened 
nucleus,  from  which  the  cytoplasm  extends  in  projections 
passing  partly  around  the  fibers.  Isolated  cementoblasts 
are  shown  in  Fig.  228,  drawn  by  Dr.  Black.  In  order  to 
obtain  an  idea  of  the  form  of  the  cementoblasts,  sections  must 
be  cut  at  a  tangent  to  the  surface  of  the  root,  and  just  miss- 
ing the  surface  of  the  cementum.  In  this  way  the  fibers  are 
cut  across  and  the  cementoblasts  are  shown  covering  the 
entire  surface  between  the  fibers.  These  are  shown  in  Fig. 
229,  in  which  the  fiUers  are  left  perfectly  clear  in  order  to 


296 


THE  PERIDENTAL  MEMBRANE 


outline  the  cells  more  distinctly.  In  sections  cut  at  right 
angles  to  the  surface  of  the  roots  (Figs.  240,  241,  and  242) 
the  cementoblasts  are  shown  as  more  or  less  flattened,  but 
no  idea  of  the  way  in  which  they  fit  about  the  fibers  can  be 
obtained. 

Cytoplasmic  processes  extend  from  the  body  of  the  cemen- 
toblasts into  the  matrix  of  the  cementum.  These  correspond 
to  the  processes  of  the  osteoblasts  which  occupy  the  canal- 
iculi  of  bone.  .  They,  however,  are  not  nearly  as  numerous 
or  as  regular  in  their  arrangement  as  the  osteoblasts.  Pro- 
cesses extending  from  these  cells  in  a  direction  from  the 
cementum  out  into  the  tissue  of  the  membrane  have  not 
been  demonstrated. 


Fig.  22S 

4  ^ 


t^^\^ 


Isolated  cemeotoblasts.  showing  the 
form  of  the  cell  as  it  fits  around  the 
fibers  springing  from  the  cementum. 


>■■■'■ 


Fig.  229 


Cementoblasts  as  seen  in  a  section 
at  a  tangent  to  the  root  and  just  miss- 
ing the  cementum.  The  fibers  are  left 
white,  the  ceils  are  shaded. 


Cement  Corpuscles. — Occasionally  a  cementoblast  becomes 
fastened  down  to  the  surface  and  enclosed  in  the  matrix 
that  is  formed.  The}^  then  lie  in  a  lacuna  and  show  processes 
radiating  from  them  into  the  canaliculi.  These  correspond 
to  bone  corpuscles,  but  there  is  no  such  regularity  of  their 
disposition  or  arrangement  with  reference  to  the  lamellae,  as 
is  shown  in  the  case  of  bone.  In  man  the  cementum  in  the 
gingi\'al  half  of  the  root  is  usually  without  cement  corpuscles. 
They  often  lie  entirely  within  a  single  lamella  instead  of 


OSTEOBLASTS  297 

between  two,  as  is  the  case  in  bone.  In  general  they  are 
found  where  the  layers  are  thick  and  the  embedded  fibers 
are  not  specially  numerous.  They  are  very  often  seen  where 
absorptions  have  been  refilled  by  the  formation  of  subse- 
quent layers  (Figs.  15-1  and  155). 

It  is  by  the  activity  of  the  cementoblasts  producing  a 
new  layer  of  cementum  that  the  fibers  are  attached  to  the 
surface  of  the  root.  In  studying  many  sections,  places  are 
found  where  the  fibers,  though  lying  in  contact  with  the 
surface  are  not  attached  to  the  cementum.  In  some  places 
it  can  be  seen  that  they  have  been  cut  off  by  absorptions. 
From  a  study  of  these  layers  it  is  evident  that  there  is  a 
constant  readjustment  in  the  attachment  of  the  fibers  to 
the  root  during  the  function  of  the  tooth,  which  probably 
adapt  it  to  slight  changes  of  position  resulting  from  wear 
and  other  conditions.  It  is  important  to  remember  that 
whenever  the  fibers  have  been  stripped  from  the  surface 
of  the  cementum,  they  can  be  reattached  to  it  only  by  the 
formation  of  a  new  layer  of  cementum,  building  the  fibers 
into  it.  This  is  certainly  possible  if  the  conditions  are 
properly  controlled,  but  the  cells  of  the  tissue  must  be  in  a 
normal  and  vitally  active  condition,  and  the  surface  of  the 
root  must  be  such  that  they  can  lie  in  physiological  contact 
with  it.  The  cure  of  a  pyorrhea  case,  therefore,  becomes  a 
biological  problem.  In  this  connection  it  is  important  to 
remember  that  a  surface  of  cementum  which  has  long  been 
bathed  in  pus  may  be  so  filled  ^\'ith  poison  that  no  cell  can 
lie  in  contact  with  it  and  perform  its  functions. 

Osteoblasts. — The  osteoblasts  of  the  peridental  membrane 
are  exactly  like  osteoblasts  in  other  positions.  They  cover 
the  surface  of  the  bone  of  the  alveolar  wall  lying  between 
the  fibers  which  are  embedded  in  it.  Even  in  the  young 
subject  they  are  not  found  in  every  position,  while  in  an 
adjoining  area  the  surface  of  the  bone  may  be  covered  with 
them.  In  the  old  subject  they  are  generally  absent  or  have 
been  reduced  to  flattened  scales,  which  are  very  difficult  to 
demonstrate;  but  even  in  these  cases  areas  will  be  found  in 
which  osteoblasts  are  present.     These  are  areas  of  active 


298 


THE  PERIDENTAL  MEMBRANE 


bone  formation.     The  osteoblasts  lay  down  bone  exactly 
as  occurs  in  attached  portions  of  the  periosteum,  but  after 


Fig.  230 


PdM 


PdB 


Penetrating  fibers  in  hone.  A  field  from  jjlute  X\':  Pil  M,  peridental  membrane; 
0¥,  osteoblasts  of  peridental  membrane;  Ob^,  osteoblasts  of  medullary  space;  Pd.B, 
solid  subperidental  and  subperiosteal  bone  with  embedded  fibers;  Ms,  medullary 
space  formed  by  absorption  of  the  solid  subperidental  bone  with  embedded  fibers; 
H.B,  Haversian  system  bone  without  fibers  built  around  the  medullary  space, 
(About  200  X) 


HB 


PLATE   XV 


HB 


PdB 


Border  of  Growing  Process. 

Cm,  eementum;  Pd,  peridental  mennbrane;  Pd.  B,  solid  subperidental  and 
subperiosteal  bone  with  enmbedded  fibers;  Ms,  medullary  space  Ibrnned  by 
absorption  of  the  solid  bone;  H.  B,  Haversian  systeni  bone  without  fibers; 
Per,  periosteuni.    (About  SO  X) 


OSTEOCLASTS  299 

a  little  thickness  of  this  solid  peridental  bone  has  been 
formed  it  is  perforated  by  penetrating  canals,  on  the  walls 
of.  which  absorptions  occur,  forming  spaces  about  which  new 
Haversian  system  bone  is  formed.  This  is  illustrated  in 
Plate  XV.  In  this  way  only  sufficient  subperidental  bone  is 
left  to  furnish  an  attachment  for  the  fibers. 

Fig.  230  shows  a  higher  magnification  of  a  small  area. 
The  osteoblasts  are  seen  between  the  fibers  on  the  surface 
of  the  alveolus,  and  the  fibers  can  be  followed  through  the 
subperidental  bone.  A  large  absorption  area  has  been  formed 
which  has  been  partly  rebuilt,  and  the  new-formed  bone 
without  embedded  fibers  is  lighter  in  color.  An  under- 
standing of  this  building  and  rebuilding  of  bone  through 
the  agency  of  the  peridental  membrane  is  necessary  to 
understand  the  development  of  the  face  and  everything  in 
connection  with  tooth  movement,  whether  physiological 
or  artificial. 

Osteoclasts. — The  osteoclasts  of  the  peridental  membrane 
are  not  constant  elements.  They  appear  and  disappear  in 
response  to  the  same  conditions  which  lead  to  their  appear- 
ance and  disappearance  in  bone.  They  are  always  large, 
multinuclear  cells,  having  from  three  or  four  to  thirty  or 
forty  nuclei  (Fig.  231).  They  may  appear  upon  the  surface 
of  the  cementum,  upon  the  surface  of  the  alveolar  wall,  or 
within  the  medullary  spaces  of  the  bone.  They  are  formed 
from  embryonal  cells  in  the  tissue  in  response  to  mechanical 
stimuli.  ^Morphologically  they  are  in  no  respect  difterent 
from  the  osteoclasts  in  bone. 

The  osteoclasts  are  tissue  destroyers  and  are  the  active 
agents  in  the  removal  of  any  hard  tissue.  There  is  no  difi'er- 
ence  in  them,  whether  they  are  destroying  the  fibrous  tissue, 
bone,  cementum,  or  dentine  (Fig.  232).  In  order  for  them 
to  act,  their  cytoplasm  must  lie  in  actual  contact  with  the 
surface  to  be  attacked.  They  do  not  first  decalcify  and  then 
remove,  but  apparently  by  applying  their  cytoplasm  to  its 
surfacejthe  cells  destroy  the  intercellular  substance,  forming 
hollows  in  the  surface,  into  which  the  cells  sink.  These 
hollows   have   been   called   Howship's   lacunae.     The   cells 


300 


THE  PERIDENTAL  MEMBRANE 


usually  appear  in  groups  and  spread  out  over  the  bone  or 
cementum  to  be  attacked,  but  sometimes  only  two  or  three 
will  be  found  at  a  point  on  the  surface  of  the  bone,  and  these 
will  burrow  into  the  substance,  forming  a  penetrating  canal 


Fig.  231 


Osteoclast   absorption   of   bone   over  permanent   tooth;   Oc,  osteoclasts;   B,  bone  of 
crypt  wall;  F,  fibrous  tissue  of  follicle  wall;   -4,  ameloblasts.      (About  62  X) 


running  through  the  bone  (Figs.  233  and  234).  In  these 
positions  the  osteoclasts  are  usually  comparatively  small.  As 
fast  as  the  canal  is  formed  the  embryonal  cells  of  the  mem- 
brane multiply  and  grow  into  the  space  and  at  any  point 


OSTEOCLASTS 


301 


where  absorption  is  going  on  the  portion  destroyed  is  immedi 
ately  replaced  by  embryonal  connective  tissue. 


Fig.  232 


Osteoclasts  in  cancellous  bone  near  the  peridental  membrane;  in  some  portions  of 
the  field  osteoblasts  are  seen.  As  bone  is  removed  note  how  embryonal  connective 
tissue  replaces  it. 

This  will  be  noted  in  all  the  illustrations  showing  absorp- 
tions.    Whenever  absorption  is  going  on  formation  is  also 


302  THE  PERIDENTAL  MEMBRANE 

going  on  in  an  adjoining  area.  In  this  way  the  function 
of  the  tissue  is  maintained  until  the  last  remnants  of  it  are 
destroyed.  The  general  statement  may  be  made  that  bone 
formation  is  always  accompanied  by  bone  destruction,  and 
bone  destruction  by  rebuilding.  The  result  depends  upon 
which  side  the  balance  swings.  The  alternation  of  formation 
and  absorption  in  the  removal  of  hard  tissues  is  well  illus- 
trated in  the  absorption  of  the  roots  of  the  temporary  teeth. 

Fig.  233 


Osteoclast  absorption  forming  penetrators  of  canal:  a,  bone  matrix;  h,  bloodves- 
sel; c,  embryonal  connective  tissue;  d,  new  bone  formation;  e,  osteoblasts,  /,  osteo- 
clasts.     (Black.) 

The  absorption  does  not  begin  at  one  point  and  spread 
continuously  over  the  entire  surface  of  the  root.  If  it  did 
so,  all  of  the  fibers  would  be  cut  out  and  the  tooth  would 
drop  off  with  at  least  a  considerable  portion  of  the  root. 
The  process  progresses  in  something  of  this  fashion.  At  a 
point  on  the  side  of  the  root  near  the  apex,  where  the  growth 
of  the  erupting  tooth  produces  pressure,  osteoclasts  appear 
in  the  membrane,  cutting  off  the  fibers,  displacing  the  cemen- 
toblasts,  and  arranging  themselves  in  groups  on  the  surface 


OSTEOCLASTS 


303 


of  the  root.  These  dissolve  away  the  cementum  and  sink 
into  the  tissue,  perhaps  cutting  into  the  dentine  for  a  short 
distance.  By  this  excavation  the  pressure  is  relieved,  the 
osteoclasts    disappear,    cementoblasts    are    formed    in    the 


Fig.  234 


A  longitudinal  section  through  the  remains  of  the  alveolar  process  around  the 
root  of  a  temporary  tooth  about  to  be  shed  (sheep):  C,  the  cementum  on  the 
remains  of   the  tooth;   B,  penetrating  canals  cut  through  the  labial  plate  of   bone. 


embryonal  connective  tissue,  and  the  deposit  of  cementum 
begins  in  the  excavation,  reattaching  the  fibers  in  this  area. 
As  the  rebuilding  progresses,  at  a  point  a  little  farther  occlu- 


Fig.  235 


Root  of  a  temporary  incisor,  sho-wing  absorption  and  rebuilding  of  cementum 
(from  sheep):  G,  gingivus;  D,  dentine;  Cm,  cementum;  Ab,  absorption  cavity, 
showing  Howship's  lacunae;   Cm'^,  new-formed  cementum.      (About  50  X) 


OSTEOCLASTS 


305 


sally,  osteoclasts  appear  and  begin  a  new  excavation.  In 
this  manner  the  process  continues.  When  the  absorption 
stops  in  the  second  point,  it  begins  again  at  the  first,  cutting 


Fig.  236 


A  transverse  section  through  an  incisor  from  the  same  jaw  as  Fig.  235,  and  at 
the  level  of  Cm^,  showing  the  refilling  of  the  absorption  cavity  by  new  layers  of 
cementum. 


much  deeper  into  the  dentine,  and,  oscillating  back  and  forth, 
it  progresses  until  all  of  the  dentine  may  be  destroyed,  leav- 
ing the  hollow  cap  of  enamel,  and  even  then  new-forming 
20 


306  THE  PERIDENTAL  MEMBRANE 

cementum  to  maintain  the  attachment  will  be  fomid  around 
the  circumference.  In  this  way  it  will  be  seen  that  the 
function  of  the  tooth  is  maintained  until  its  successor  is 
ready  to  take  its  place  in  a  very  short  time.  The  importance 
of  this  arrangement  will  be  more  fully  appreciated  after  a 
study  of  the  relation  of  the  teeth  to  the  development  of  the 
face.  Fig.  235  shows  a  longitudinal  section  through  a 
temporary  incisor  of  a  sheep.  At  Ah  an  absorption  has 
just  been  completed,  for  the  osteoclasts  have  disappeared. 
The  excavation  is  seen  filled  with  embryonal  tissue  and 
rebuilding  is  about  to  begin.  At  Cm  an  older  and  much 
larger  absorption  space  is  seen  which  has  been  partially 
replaced  by  a  formation  of  new  cementum  reattaching 
fibers.  In  Fig.  23G  a  transverse  section  of  the  root  is  seen 
which  is  from  the  same  jaw  cut  at  the  level  of  Cm,  and  shows 
the  absorption  refilled.  This  patchwork  performance  goes 
on  in  the  same  way  in  the  bone  of  the  alveolar  process,  and 
its  study  is  one  of  the  most  interesting  phases  of  the  rela- 
tion of  the  teeth  to  the  development  of  the  face.  Without  a 
clear  idea  of  this  it  is  impossible  to  understand  how  the 
teeth,  after  their  roots  are  fully  formed,  can  move  through 
three  dimensions  of  space  and  retain  their  function  all  the 
time. 

Epithelial  Structures.— The  epithelial  structures  of  the 
peridental  membrane  were  first  described  by  Dr.  Black  in 
his  volume  Periosteum  and  Peridental  Memhrane,  pub- 
lished in  1887.  At  this  time  Dr.  Black  considered  them  to 
be  of  lymphatic  character  and  named  them  endolymphatics. 
His  conception  of  them  was  that  they  were  lymphatic 
channels  crowded  with  adenoid  cells.  Since  then  the  form 
and  appearance  of  the  cells  and  the  character  of  their 
reaction  with  staining  agents  has  shown  the  cells  to  be  of 
epithelial  character.  In  the  same  year  that  Dr.  Black's 
book  was  published,  von  Brunn^  described  the  same  struc- 
tures. He  considered  them  as  epithelial  remains  of  the 
outer  layer  of  the  enamel  organ,  growing  down  around  the 

'ArcLiv  f.  Anatomie,  1887. 


DISTRIBUTION  307 

root  beyond  the  gingival  line  where  the  formation  of  enamel 
stops.  It  has  seemed  probable  to  the  writer  that  this  was 
correct,  but  their  histogenesis  has  not  been  sufficiently  well 
followed,  and  it  presents  an  attractive  field  for  research. 
Although  this  is  the  origin  of  these  structures,  it  has  never 
seemed  proper  to  regard  them  as  embryonal  remains,  for 
while,  like  all  the  cellular  elements,  they  are  more  numerous 
in  young  people  than  in  old,  they  are  persistent  throughout 
life.  They  have  been  shown  in  the  membrane  from  a  man 
aged  seventy  years,  and  it  does  not  seem  logical  to  suppose 
that  embryonal  debris  that  was  useless  to  the  organism 
would  persist  through  life.  Up  to  the  present  time,  however, 
nothing  has  been  discovered  about  these  structures  to  throw 
any  light  upon  their  function.  Specimens  have  strongly 
indicated  that  they  were  important  in  some  pathologic 
conditions.  Their  cells  have  been  found  dead  and  degener- 
ating in  pathologic  material  beyond  the  point  showing  any 
pathologic  condition  in  other  cells.  These  structures 
have  been  observed  in  sections  from  man,  sheep,  cat,  dog, 
and  monkey.  The  best  material  for  their  study  is  a  young 
sheep  or  pig. 

Distribution. — These  structures  are  composed  of  cords  or 
rows  of  epithelial  cells,  surrounded  by  an  extremely  delicate 
basement  membrane  (Fig.  2.37).  In  some  cases  there  is  a 
slight  indication  of  a  circular  arrangement  of  connective 
tissue  around  them.  The  cords  lie  very  close  to  the  surface 
of  the  cementum,  winding  in  and  out  anjong  the  fibers 
(Fig.  238).  They  anastomose  and  join  with  each  other, 
forming  a  network  the  meshes  of  which  are  comparatively 
close  in  the  gingival  portion  (Fig.  239),  and  comparatively 
wide  in  the  apical  portion,  the  cords  becoming  scarcer  as  the 
apex  of  the  root  is  approached,  but  the  author  has  seen  them 
in  sections  from  the  apical  third. 

A  binocular  microscope  was  used  to  obtain  a  true  concep- 
tion of  the  way  in  which  these  cords  wind  in  and  out  among 
the  bundles  of  fibers.  The  cords  show  a  marked  tendency 
to  run  out  into  the  membrane  and  loop  back  (Fig.  240), 
coming  very  close  to  the  surface  of  the  cementum. 


308 


THE  PERIDENTAL  MEMBRANE 


The  ends  of  the  loops  toward  the  cementum  often  show 
enlargements  which  in  some  cases  apparently  lie  directly 
in  contact  with  the  cementum  (Figs.  241  and  242).  These 
enlargements  next  to  the  cementum  are  shown  in  Fig.  240. 

The  Arrangement  of  the  Cells. — There  is  no  definite  arrange- 
ment of  the  cells  in  these  cords.  In  some  places  there  will  be 
a  ring  of  irregular  polyhedral  or  rounded  cells  which  almost 
exactly  resemble  a  simple  tubular  gland.  In  other  places 
there  is  a  pretty  definite  outer  ring  of  cells  and  a  central 


Fig.  237 


^s^ 


M 


Diagram  of   glands  of   peridental  membrane.      (Black.) 


mass  enclosed  by  them.  The  cells  are  made  up  of  granular 
cytoplasm,  each  containing  an  oxoid  nucleus  that  is  rich  in 
chromatin.  The  author  has  spent  much  time  attempting 
to  work  out  the  relation  of  these  cords  to  the  epithelium 
lining  the  gingival  space,  thinking  that  possibly  they  open 
into  it.  In  a  few  places  structures  appearing  very  much 
like  a  duct  have  been  seen,  as  shown  in  Fig.  244,  but  they 
are  apparently  only  unusually  large  cords.  There  is  no 
regularity  in  places  where   they  are   found,  and   no  con- 


A  sectioEL  cutting  diagonally  through  the  root,  showing  the  network  of  epithelial 
cords,   A;  dentine,  D;   cementum,  Cm. 


310 


THE  PERIDENTAL  MEMBRANE 


nection  with  the  gingival  space  has  ever  been  discovered. 
Toward  the  gingival,  as  the  gingival  line  is  approached,  the 
cords  seem  to  swing  out  away  from  the  cementum,  espe- 
cially on  the  proximal  side,  and  to  pass  up  into  the  gingivus, 
where  they  are  lost  among  the  projections  of  the  epithelium. 


Fig.  239 


Transverse   section  of   the  perideni 
the  position  of   the  epithelial    cords. 
in  Fig.  241  is  seen. 


il    membrane  in  the  gingival  portion,  showing 
At  1  the  loop   shown  in  higher  magnification 


Gland  of  Serres. — Salter,  in  his  Dental  Pathology  and  Sur- 
gery, quotes  Serres,  who  assigns  the  function  of  a  gland  to  the 
epithelium  lining  the  gingival  space.  This,  the  writer  believes, 
is  the  first  reference  to  an  appearance  in  the  tissues  that  has 
been  called  the  gland  of  Serres.  It  has  long  been  noted  that 
the  epithelium  lining  the  gingival  space  was  lighter  in  struc- 
ture, composed  of  larger  cells,  and  had  no  horny  layer  on  its 


GLAND  OF  SERRES 


311 


surface,  as  is  true  of  the  epithelium  on  the  outer  surface  of 
the  gingivus.     Upon  the  proximal  surfaces  the  projections 


Fig.  240 


Epithelial  structures  of  the  peridental  membrane  (from  sheep):  Fb,  fibroblasts; 
Ec,  epithelial  structures;  Cb,  cementoblasts;  Cm,  cementum;  D,  dentine.  (About 
468  X) 


of  the  epithelium  which  extend  down  between  the  papilla^  of 
connective  tissue,  which  constitute  the  stratum  papillaris, 
are  specially  long,   and  in  the  connective  tissue  between 


312 


THE  PERIDENTAL  MEMBRANE 


them  collections  of  small  round  cells  are  often  found.  It 
is  between  these  projections  of  epithelium  that  the  cords  of 
epithelial  cells  which  have  been  described  are  lost,  and  to 
this  portion  of  the  tissue  Dr.  Black  has  again  called  attention, 


Fig.  241 


Epithelial  structures:  Ec,  epithelial  cord,  apparently  showing  a  lumen;   Ch,  cemento- 
blasts;  Cm,  cementum;  D,  dentine.     This  loop  is  seen  in  Fig.  226. 


as  the  gland  of  Serres.  Sufficient  work  has  not  xo^t  been 
done  upon  this  subject  to  know  whether  this  is  a  constant 
arrangement,  or  whether  it  is  found  only  in  certain  animals, 
or  even  whether  it  may  not  possib'y  be  pathologic.  The 
appearance  is  shown  in  Plate  XVI  and  Figs.  245  and  246. 


BLOODVESSELS 


313 


Bloodvessels. — The  peridental  membrane  possesses  a  very 
rich  blood  supply.  A  number  of  vessels  enter  the  membrane 
in  the  apical  portion  from  the  medullary  spaces  in  the  bone. 


Fig.  242 


Transverse   section,  showing   the   cellular   elements:    Fb,   fibroblasts;   Ec,  epithelial 
structures:   Cb,  cementoblasts;   Cm,  cementum;  D,  dentine,      (About  900  X) 


Some  of  these,  passing  through  canals  in  the  apex  of  the  root, 
supply  the  dental  pulp,  others  pass  up  through  the  mem- 
brane. As  they  extend  occlusally  they  give  off  and  receive 
branches  which  enter  the  membrane  from  the  bone  of  the 


:U4 


THE  PERIDENTAL  MEMBRANE 


alveolar  wall.  In  this  way  the  caliber  of  the  principal  vessels 
is  maintained  throughout  their  course  in  the  membrane.  As 
they  reach  the  border  of  the  alveolar  process  they  give  oflF 


Fig.  243 


Epithelial    structures    (from    sheep):    /'7),  fibrohhists;    /w,  ei)itheHal    structures;    Cb, 
cementoblasts;   Cm,  cementuni;   D,  dentine.      (About  700  X) 


branches  which  anastomose  with  the  vessels  of  the  perios- 
teum and  gum  tissue.  These  are  shown  in  Plate  XVII. 
In  the  young  membrane  these  vessels  occupy  a  position 


A 


m: 


Wp^^^ 


\  I 


r 
> 

X 

< 


"^^^' 


BLOODVESSELS  315 

closer  to  the  bone  than  the  cementiim,  and  as  the  membrane 
becomes  thinner  they  often  come  to  He  in  grooves  in  the  bone. 
Vessels  of  any  size  are  rarely  seen  close  to  the  cementum,  and 
the  capillaries  in  the  membrane  are  rather  scarce,  though 
they  are  more  numerous  than  in  most  connective  tissues  of 
as  compact  a  character.  The  anastomosis  of  the  vessels  in 
the  membrane  is  quite  rich.     It  is  important  to  remember 

Fin.  244 


A  very  large  cord  which  was  at  first  mistaken  for  a  duct. 


that  the  cancellous  bone  of  the  process  is  richly  supplied  with 
bloodvessels,  and  the  anastomosis  with  the  vessels  of  the 
membrane,  from  the  alveolar  wall  and  over  the  border  of 
the  process,  is  important  in  the  consideration  of  pathologic 
conditions.  In  alveolar  abscess  the  vessels  entering  through 
the  apical  space  ma}'  be  entirely  cut  ofl',  but  this  does  not 
disturb  the  blood  supply  of  the  rest  of  the  membrane.  The 
removal  of  the  pulp  has  often  been  advocated  in  the  treatment 


316 


THE  PERIDENTAL  MEMBRANE 

Fia.  245 


Longitudinal  section,  cut  mesiodistally,  similar  to  Plate  XVI:  D,  dentine;  Cm, 
cementum  which  has  separated  from  the  dentine;  Gs,  gingival  space;  Ep,  epithelial 
projection  from  the  lining  of  the  gingival  space,  Ec,  epithelial  cords;  Rs,  small 
round  cells  in  the  connective  tissue. 


BLOODVESSELS 


317 


Fig.  24G 


A  longitudinal  section  cut  mesiodistally:  E,  epithelium  of  the  gingivus;   Gs,  gingivs 
space;  Cm,  cementum  -which  has  separated  from  the  dentine;  Ec,  epithelial  cords. 


318  THE  PERIDENTAL  MEMBRANE 

of  pathologic  conditions  of  the  membrane,  on  the  ground 
that  the  vessels  entering  the  pulp  rob  the  membrane  of 
blood  supply,  and  that  their  removal  made  recovery  more 
certain.  No  one  having  a  knowledge  of  the  blood  supply  of 
the  membrane  could  advise  this  for  this  reason. 

In  their  course  through  the  membrane  the  vessels  wind 
between  the  principal  fibers  in  a  way  that  can  only  be  appreci- 
ated by  studying  sections  with  a  binocular  instrument,  and 
when  this  condition  is  realized  it  can  be  understood  how  some 
inflammations  in  the  membrane  are  set  up.  For  instance,  when 
force  is  applied  to  a  tooth,  the  principal  fibers  are  stretched. 
This  causes  them  to  close  some  spaces  and  open  others. 
The  vessels  in  the  closed  spaces  are  constricted  and  the  flow 
of  blood  through  them  partly  shut  off.  The  vessels  in  the 
enlarged  spaces  dilate  to  compensate.  If  the  force  is  removed, 
the  dilated  vessels  are  again  constricted,  and  the  constricted 
ones  enlarged,  and  the  result  is  a  literal  sawing  upon  the 
walls  of  the  bloodvessels  which  in  a  very  short  time  will  set 
up  an  acute  inflammation.  This  is  extremely  important  in 
the  application  of  force  in  orthodontia,  and  often  also  in  the 
use  of  the  mallet  in  condensing  gold,  especially  for  young 
patients. 

Nerves. — The  nerves  of  the  peridental  membrane  enter 
the  peridental  membrane  in  company  with  the  bloodvessels. 
Their  source  is  the  same  as  that  of  the  bloodvessels.  The 
trunks  entering  in  the  apical  space  contain  from  eight  or 
ten  to  fifteen  or  twenty  medullated  fibers.  Some  of  these 
enter  the  dental  pulp,  others  extend  up  through  the  mem- 
brane, winding  in  and  out  among  the  fibers,  generally 
following  the  course  of  the  bloodvessels.  Many  trunks 
containing  eight  or  ten  fibers  enter  through  the  alveolar  wall. 
In  this  way  a  fairly  rich  plexus  is  formed,  from  which  fibers 
are  continually  given  off  to  be  lost  in  the  tissue.  They 
probably  terminate  in  beaded  free  endings.  No  special 
nerve  endings  have  been  demonstrated.  A  few  Pacinian 
corpuscles  have  been  seen  near  the  gingival  border.  These 
are  not  generally  found,  however.  The  nerves  of  the  mem- 
brane give  to  it  the  sense  of  touch,  which  is  the  only  sensory 


CHANGES  IX  PERIDENTAL  MEMBRANE  WITH  AGE     319 

function  of  the  membrane.  As  has  been  noted  in  connec- 
tion with  the  dental  pulp,  the  hard  tissues  and  the  pulp  have 
no  sense  of  touch.  The  contact  of  any  substance  with  the 
surface  of  the  tooth  is  reported  to  consciousness  through 
the  medium  of  the  peridental  membrane.  For  instance,  the 
slightest  touch  of  a  delicate  instrument  produces  a  slight 
movement  of  the  tooth  which  affects  the  nerves  between 
the- fibers.  The  delicacy  of  this  mechanism  can  be  demon- 
strated by  the  following  experiment.  Lightly  touch  the 
surface  of  the  enamel  and  the  patient  will  tell  at  once  not 
only  which  tooth  is  touched,  but  whether  a  steel  instrument 
or  a  wooden  point  or  some  soft  material  was  used.  If, 
however,  the  finger  is  placed  upon  a  surface  of  the  tooth  and 
firm  pressure  made  in  one  direction,  the  contact  of  the  point 
will  not  be  recognized. 

The  Changes  in  the  Peridental  Membrane  with  Age. — The 
teeth  are  formed  in  crypts  in  the  bone,  and  when  they  begin 
to  erupt  the  roof  of  the  crypt  is  removed  by  absorption, 
making  an  opening  large  enough  for  the  crown  to  pass.  As 
the  root  is  formed,  the  tooth  moves  occlusally  and  the 
alveolus  grows  up  around  it,  beginning  at  the  margins  of  the 
crypt.  When  the  tooth  first  erupts,  therefore,  the  alveolus 
is  much  larger  than  the  root,  and  the  fibers  of  the  peridental 
membrane  are  very  long.  The  size  of  the  alveolus  is  reduced 
by  the  formation  of  bone,  by  the  osteoblasts  on  its  wall,  and 
the  size  of  the  root  is  increased  by  the  formation,  layer  after 
layer,  on  its  surface.  In  this  way  the  thickness  of  the  mem- 
brane is  reduced.  Figs.  247  and  248  were  made  to  illustrate 
this  change.  They  were  photographed  with  as  nearly  the 
same  magnifications  as  possible,  so  as  to  compare  the  thick- 
ness of  the  layers.  In  the  first,  there  are  but  two  layers  of 
cementum;  notice  the  thickening  of  the  last-formed  layer, 
to  attach  the  strong  bundles  of  fibers  at  the  angle  of  the  root. 
The  second  is  from  a  temporary  tooth  which  has  been  in 
position  and  function  for  a  long  time;  notice  the  thickness 
of  the  cementum  and  that  the  formation  of  bone  and  cemen- 
tum has  reduced  the  thickness  of  the  membrane  to  not  more 
than  one-third  of  its  original  amount.    Notice  also  that  the 


320 


THE  PERIDENTAL  MEMBRANE 


surface  of  both  bone  and  cementum  are  not  even,  but 
scalloped,  and  that  where  the  cementum  projects  toward 
the  alveolar  wall  there  is  a  depression  in  the  bone,  and  where 
the  bone  projects  toward  the  cementum  there  is  a  depression 
in  the  cementum.    There  is,  therefore,  a  distinct  tendency 


Young  membranes  (from  sheep):  D,  dentine;  Cm,  cementum;  C'mi,  thickening  of 
cementum  to  attach  fibers  at  the  corner;  Pd,  peridental  membrane;  B,  bone  forming 
the  wall  of  the  alveolus,      (About  80  X ) 


for  the  two  tissues  to  interlock  but  remain  separated  by  a 
layer  of  fibrous  tissue.  The  author  has  never  seen  a  specimen 
showing  a  union  between  calcified  substances  of  bone  and 
cementum.  Two  surfaces  of  cementum  may  become  united 
by  direct  calcification  and  the  teeth  fused  together.     This 


PRACTICAL  CONSIDERATION 


321 


is  illustrated  in  many  freak  specimens  to  be  found  in  any 
dental  museum.  It  is  often  stated  that  a  tooth  had  become 
ankylosed  to  the  bone,  but  to  the  author's  knowledge  no 
specimen  has  ever  been  shown  in  which  the  separating 
layer  of  fibrous  tissue  was  not  present. 


Fig.  248 


Old   membranes  (from  sheepi:    D,  dentiue:    Cm,  cemeuiuiu;    Pd,  pendeutal  mem- 
brane;  5,  bone  forming  the  wall  of  the  alveolus;  P,  pulp.      (About  80X) 


Practical  Consideration. — These  structural  facts  are  of  the 
greatest  practical  importance,  especially  in  the  making  of 
gold  fillings  for  young  persons.  Every  operator  has  noticed 
the  greatest  difference  in  the  feeling  of  the  instrument  under 
the  mallet  upon  difterent  teeth.  In  one  instance  it  will  ring 
under  the  steel  mallet  as  if  the  tooth  were  resting  on  an  anvil; 
21 


322  THE  PERIDENTAL  MEMBRANE 

ill  {uiotlier  case  it  feels  as  if  the  tooth  were  resting  on  a 
cushion.  In  the  first  case  all  of  the  force  of  the  blow  is 
expended  in  the  condensation  of  the  gold.  In  the  second, 
a  large  proportion  is  lost  in  the  movement  of  the  tooth. 
If  the  membrane  is  thin  and  the  cementum  and  bone  are 
interlocked,  the  tooth  is  firmly  supported.  If  the  membrane 
is  thick  and  the  fibers  long,  as  in  the  first  illustration,  the 
blow  is  dissipated  in  the  sag  of  the  fibers.  The  tooth  is 
jumping  up  and  down  in  its  socket.  The  force  used  is 
dissipated,  the  gold  is  not  condensed,  and  in  a  very  short  time 
an  acute  inflammation  is  set  up  and  the  tooth  becomes  very 
sore  to  the  blows.  This  the  author  believes  is  the  explanation 
of  the  idea  that  gold  will  not  preserve  teeth  for  young 
children.  It  has  often  been  said  that  children's  teeth  are 
too  soft  for  gold  fillings.  The  difficulty  is  not  with  the 
enamel  and  dentine,  but  because  of  the  thickness  of  their 
membranes.  The  gold  is  not  sufficiently  condensed  to 
exclude  moisture,  and  the  fillings  fail.  Serious  damage  also 
may  be  done  to  the  membrane.  The  Museum  of  the  North- 
western University  Dental  School  contains  an  object  lesson 
on  this  point.  It  consists  in  a  bicuspid  with  a  beautifully 
condensed  and  finished  gold  filling,  in  a  mesial  occlusal  cavity. 
The  history  accompanying  it  is  somewhat  as  follows:  The 
operation  was  undertaken  for  a  patient  aged  about  fourteen 
years.  The  tooth  became  exceedingly  sore  under  the  mallet; 
the  filling  was,  however,  completed  and  polished,  but  a  few 
days  later  the  tooth  was  picked  out  with  the  fingers.  The 
peridental  membrane  had  been  literally  hammered  to  death. 
Stated  in  scientific  terms,  the  fibers  had  sawed  upon  the 
bloodvessels,  exciting  an  acute  inflammation,  resulting  in 
complete  stasis  and  the  death  of  the  tissue.  In  all  opera- 
tions where  gold  is  to  be  condensed  in  teeth  with  thick 
membranes  they  must  be  firmly  supported  so  as  to  be  held 
rigidly  against  the  blow. 


CHAPTER  XXIV 


THE   IMOUTH   CAVITY 


Mucous  Membrane. — The  mucous  membrane  lining  the 
mxouth  cavity  is  composed  of  a  layer  of  stratified  squamous 
epithelium  supported  upon  a  tunica  propria,  which  is  usually 
described  as  composed  of  two  parts^ — the  papillary  layer 
and  the  reticular  layer.  The  epithelium  and  the  tunica 
propria  make  up  the  mucous  membrane  proper,  which  is  sup- 
ported upon  a  submucous  layer  composed  of  a  coarse  network 
of  white  and  elastic  fibers,  containing  the  larger  bloodvessels. 

The  Epithelium. — The  stratified  squamous  epithelium  is 
provided  with  a  horny  or  corneous  layer  only  in  the  por- 
tions covering  the  alveolar  process  and  the  hard  palate,  or, 
in  other  words,  where  the  submucosa  is  firmly  attached  to 
the  periosteum  (Fig.  249).  In  these  positions  the  horny 
layer  consists  of  dead  cells  which  have  lost  their  nuclei 
and  whose  cytoplasm  has  been  converted  into  keratin  or 
horny  material. 

These  scale-like  cell  remains  are  closely  packed  into  a 
protective  layer.  There  is  no  distinct  stratum  lucidum 
separating  the  dead  from  the  living  cells,  as  there  is  in  the 
skin.  In  the  deeper  portions  the  cells  possess  oval  or  rounded 
nuclei  and  become  larger  and  more  polyhedral  as  the  base- 
ment membrane  is  approached.  The  cells  of  the  deepest 
layer  next  to  the  basement  membrane  are  tall  and  approach 
the  columnar  form,  but  are  never  much  greater  in  height 
than  width.  The  deep  layer  is  often  called  stratum  ]\Ial- 
pighii.  The  epithelium  lining  the  gingival  space  and  that 
coverings  unattached  portions  is  without  the  horny  laj'er, 
and  the  cells  are  larger  and  more  loosely  placed.  The  poly- 
hedral cells  in  the  middle  portion  of  the  layer  show  distinct 


324 


THE  MOUTH  CAVITY 


intercellular   spaces   across   which   the   cytoplasm   extends 
in  intercellular  bridges. 

Isolated  cells  from  this  region  show  the  broken  bridges 
projecting  from  their  surface,  and  for  this  reason  have  been 
called  "pickle  or  prickle  cells."  In  these  positions  the  thick- 
ness of  the  epithelial  layer  is  usually  greater  than  in  the 
attached  portions  of  the  membrane  (Fig.  250). 

Fig.  249 


Stratified    squamous   epithelium  covering   the    alveolar   process:    C,  corneous   layer; 
P,  papilla  of   connective  tissue.      (About  400  X) 


Tunica  Propria. — The  connective-tissue  layer  of  the  mucous 
membrane  interlocks  with  the  epithelial  layer  by  means  of 
the  tunica  papillaris,  which  is  composed  of  very  delicate 
white  and  elastic  connective-tissue  fibers.  They  are  usually 
about  half  as  tall  as  the  thickness  of  the  epithelium,  and 
about  one-third  as  wide  as  they  are  tall.  The  height  and 
character  of  the  papillae  varies  greatly,  however,  in  different 
positions.    In  the  red  border  of  the  lip  and  in  the  epithelium 


GLANDS  OF  THE  SUB  MUCOSA 


325 


lining  the  gingival  space  they  are  very  tall  and  narrow,  and 
approach  very  close  to  the  surface  of  the  epithelium.  Over 
the  gums  and  the  palate  they  are  much  shorter  and  wider  and 
do  not  extend  more  than  half-way  through  the  epithelium. 
These  papilla?  contain  loops  of  capillary  bloodvessels,  and 
in  some  special  nerve  endings  are  found. 

Fig.  250 


Stratified  squamous  epithelium  from  unattached  mucous  membrane  of  the  mouth. 
The  corneous  layer  is  absent.      (About  200  X) 

Reticular  Layer. — The  reticular  layer  joins  the  papillary 
layer  without  any  line  of  demarcation,  and  is  composed  of 
the  same  kind  of  tissue,  the  fibers  being  arranged  in  a  deli- 
cate network.  Everywhere  in  the  tunica  propria  are  found 
ducts  from  mucous  glands  which  lie  in  the  deeper  layers. 

Submucosa. — The  submucosa  is  composed  of  firm  connec- 
tive tissue  in  which  the  white  fibers  are  in  large,  strong 
bundles,  and  elastic  fibers  are  scarce.  It  contains  two 
plexuses  of  bloodvessels,  both  more  or  less  parallel  with 
the  surface.  The  outer  is  composed  of  small  vessels  forming 
a  smalk  meshed  network,  the  deeper  of  large  vessels  more 
\\idely  separated.  Lymphatic  vessels  everywhere  follow 
the  course  of  the  bloodvessels. 


326 


THE  MOUTH  CAVITY 


Glands  of  the  Submucosa. — The  siibmiicosa  contains  a  great 
many  small  tubular  glands.  These  are  distributed  widely 
over  the  tongue  and  membrane  of  the  cheek  and  lip  (Fig. 
251).    They  are  branched  tubular  glands,  sometimes  simple 


Fig.  251 


k. 


Mucous 
(jkmd' 


Epitlielinm  [ 
of  mucous. -.L 
memhrane 


Blood- 

vessplfi 


Epithelium 
of  mucous 
membrane    \/ 


Cross  sections^ 
of  muscle'-~-\{-~. 


P- 


'^O.  Ffnir 

■  ]\     '  foJIicles 

-yr—Fnt 


-f-'Epidermis 


f  ^,(ll (1)1(1  s 


_  _  -  -  -       ^  of  muscle 
ibres 


Mucous 
membrane 
tvith  high'' 

papillse  " 


Longitudinal 
sections 


Place  irhere  stratum 
corneum  begins 


Section  through  the  upper  lip  of   a  two-and-a-half-j^ear-old  child.      (14  X) 


THE  TONGUE 


327 


and  sometimes  compound.  The  body  of  the  gland  is  always 
in  the  submucosa,  though  it  may  extend  into  the  underlying 
muscle.  Some  are  serous  and  others  mucous,  while  many  of 
the  larger  ones  contain  cells  of  both  types.  The  secretion 
of  these  glands  is  probably  much  more  important  than  has 
been  supposed. 

Nerve  Endings  in  the  Mucosa. — Sensory  nerve  endings  of 
two  kinds  are  found  in  the  mucous  membrane.  Krause's 
end  bulbs  are  found  in  many  of  the  papillae,  and  other  nerves 
terminate  in  free  endings  lying  between  the  epithelial  cells. 


Fig.  252 


A  section  from  the  side  of  the  tongue:   E,  epithelium;   Sin,  submucosa;   Bv,  blood- 
vessels;  Af,  muscle  fibers;   G,  mucous  glands. 


The  Tongue. — The  tongue  is  composed  of  a  mass  of  volun- 
tary muscle  fibers  arranged  in  complicated  interlacing  bun- 
dles, covered  by  the  mucous  membrane.  The  most  striking 
characteristics  of  the  mucous  membrane  of  the  tongue  (Fig. 
252)  are:  (1)  The  thinness  of  the  submucosa,  which  holds 


328  THE  MOUTH  CAVITY 

it  closely  to  the  mass  of  muscle  and  allows  very  little  move- 
ment of  it;  (2)  the  submiicosa  in  the  dorsal  surface  contains 
no  glands,  though  there  are  glands  among  the  muscle  fibers 
whose  ducts  pass  through  the  submucosal  (3)  the  presence 
of  the  epithelial  papilla  upon  its  dorsal  surface.  The  tongue 
is  imperfectly  divided  vertically  on  the  median  line  by  a  band 
of  connective  tissue  forming  the  median  raphe  or  septum, 
which  causes  the  depression  at  the  central  line  of  the  dorsal 
surface. 

The  Muscles. — The  muscles  of  the  tongue  include  two 
groups — the  extrinsic  and  the  intrinsic.  The  extrinsic  mus- 
cles comprise  the  genioglossus,  the  hyoglossus,  the  styloglos- 
sus, and  the  palatoglossus.  These  are  all  paired  and  extend 
from  the  skull  or  the  hyoid  bone  into  the  tongue.  The 
intrinsic  muscles  comprise  the  principal  muscles  of  the 
tongue,  the  lingualis.  A  transverse  section  through  the 
body  of  the  tongue  in  the  central  portion  shows  a  compli- 
cated network  of  muscle  fibers  running  in  three  directions — 
longitudinally,  transversely,  and  vertically.  The  longitu- 
dinal fibers  are  arranged  around  the  outer  portion,  forming 
a  cortical  layer  about  5  mm.  thick.  These  constitute  the 
chief  bulk  of  the  lingualis,  supplemented  by  fibers  from  the 
styloglossus.  The  vertical  fibers  are  mostly  deeply  placed 
in  the  central  portion  on  either  side  of  the  raphe.  They  are 
chiefly  derived  from  the  genioglossus  and  radiate  toward  the 
dorsal  surface.  The  transverse  fibers  are  entirely  from  the 
lingualis  except  for  a  few  from  the  palatoglossus.  They  arise 
from  the  septum  and  interlace  with  the  longitudinal  and 
vertical  fibers.  They  break  up  into  strands  running  between 
the  longitudinal  fibers  of  the  cortical  portion,  and  spread 
out  to  a  submucous  insertion. 

The  complicated  movements  of  the  tongue  are  accom- 
plished by  the  contractions  of  these  sets  of  muscles.  When 
the  longitudinal  fibers  are  relaxed  and  the  transverse  fibers 
contracted  the  tongue  is  rolled  and  extended.  When  the 
transverse  fibers  are  relaxed  and  the  vertical  fibers  contracted 
the  tongue  is  flattened.  The  division  of  the  tongue  on  the 
median  line  by  the  septum  allows  each  half  to  work  inde- 


THE  PAPILLA 


329 


pendently,  so  that  when  the  longitudinal  fibers  are  con- 
tracted on  one  side  and  relaxed  on  the  other  the  tip  of  the 
tongue  is  moved  sideways. 

The  Papillae. — The  roughness  of  the  dorsal  surface  of  the 
tongue  is  caused  by  projections  of  the  epithelium  resting 
upon  the  tunica  propria,  forming  the  papillae  of  the  tongue. 
These  projections  are  not  to  be  confused  with  the  connective- 

FiG.  253 


Mucous  membrane  from  the  dorsal  surface  of  the  tongue  of  a  kitten,  showing 
filiform  and  fungiform  papillse. 


tissue  papillae  in  the  tunica  propria  of  the  mucous  membrane. 
They  are  of  three  kinds — the  filiform  and  fungiform  papillae, 
which  are  found  over  the  entire  dorsal  surface,  and  the 
circumvallate  papillae,  which  are  limited  in  number  and 
confined  to  the  posterior  portion.  The  filiform  are  much 
the  more  numerous,  especially  near  the  tip  of  the  tongue. 
They  are  from  0.5  to  2.5  mm.  in  height,  and  often  end  in 
brush-like  strands  of  epithelial  cells. 


3:^0 


THE  MOUTH  CAVITY 


The  fungiform  papillse  form  the  red  points  on  the  surface 
of  the  tongue,  especially  near  the  edges,  because  of  the 


Fig    254 


Mucous  membrane  from  the  tongue  of  a  rabbit,  showing  circum vallate  papills 
with  taste  buds  on  their  sides. 


Fig.  255 

P  h 


s 

A  section  of  a  taste  bud:     p,   pore;  g,   gustatory   cells;  ep,   epithelial   cells; 
s,  sustentacular  cells;  h,  bristles  of  the  gustatory  cells.    (Schaefer.) 


THE   TONSIL  331 

thinness  of  their  epithelium.  They  are  low  and  rounded  in 
form,  from  0.5  to  1.5  mm.  in  height,  and  are  named  from 
their  mushroom-like  appearance.  Fig.  253,  a  section  from 
the  tongue  of  a  kitten,  shows  the  form  of  both  of  these 
papillae.  The  circumvallate  papillae  usuall}^  number  nine 
or  ten,  and  are  arranged  in  a  V-shaped  form  near  the  base 
of  the  tongue,  with  the  apex  extending  backward.  They  are 
from  1  to  1.5  mm.  in  height  and  from  2  to  3.5  mm.  in  width. 
They  are  surrounded  by  a  depression,  so  that  the  upper 
surface  of  the  papillae  is  not  much  above  the  general  level 
of  the  membrane. 

The  Taste  Buds.— These  are  found  chiefly  on  the  sides  of  the 
circumvallate  papillae  (Fig.  254),  though  they  are  occasionally 
found  in  the  epithelium  of  the  fungiform  papillae  and  the  soft 
palate,  and  on  the  posterior  surface  of  the  epiglottis.  They 
are  always  entirely  embedded  in  the  epithelium  and  extend 
through  its  entire  thickness.  The  structures  are  ovoid  in  form, 
with  the  rounded  end  toward  the  connective  tissue  and  the 
pointed  end  at  the  surface,  where  a  small  opening,  the  taste 
pore,  communicates  with  the  mouth  cavity  (Fig.  255).  INIost 
of  the  cells  are  elongated  and  spindle-shaped,  and  arranged 
like  the  leaves  of  an  onion.  Four  varieties  may  be  recog- 
nized. The  outer  sustentacular  cells  form  the  outer  layer 
and  are  in  contact  with  the  epithelial  cells.  They  are 
elongated,  with  an  oval  nucleus  near  the  centre.  The  inner 
sustentacular  are  rod-shaped  cells,  more  slender  in  form, 
with  a  nucleus  at  the  base.  The  neuro-epithelial  cells  are 
elongated,  spindle-shaped  cells  at  the  centre  of  the  taste  bud. 
The  nucleus  is  at  the  base  of  the  cell,  and  from  the  opposite 
end  a  stiff  bristle-like  process  extends  through  the  taste  pore. 

The  basal  cells  are  irregular  in  form  with  large  oval  nuclei; 
they  communicate  with  each  other  and  the  sustentacular 
cells  by  cytoplasmic  bridges.  They  form  the  base  of  the 
taste  bud.  The  function  of  the  taste  buds  is  probably 
related  to  the  function  of  deglutition  rather  than  the  sensa- 
tion of  taste. 

The  Tonsil. — In  the  posterior  part  of  the  tongue  and  the 
wall  of  the  pharynx  is  found  adenoid  tissue  in  the  form  of 


332 


THE  MOUTH  CAVITY 


solitary  follicles  lying  in  the  tunica  propria  and  invading 
the  epithelium.  This  adenoid  tissue  forms  an  organ  which 
Waldeyer  has  called  the  lymphatic  pharyngeal  ring.  This 
tissue  is  divided  into  three  main  masses — that  lying  in  the 
base  of  the  tongue  forming  the  lingual  tonsil,  that  associated 
with  the  palate  and  lying  between  the  pillars  of  the  pharynx 
and  forming  the  palatine  tonsil,  and  that  situated  in  the 
pharynx  or  pharyngeal  tonsil. 


.^- 


.^, 


Epithe- 
lium 


TanicK 

propria 

Lympli 

nodule 


Oblique 

section  "j"'".-      .^c;i^. 
of  duct — p--^  -  -' 
of  raucous 
gland 

Muscle 


fibres 
cut — ii^. 


trans- 
versely 


\^ar<rXZ- 


Adenoid 
tissue 


Section  through  a  Ungual  foUicle  in  man:  x,  crypt.     (50  X)      (Szymonowicz.) 


The  Lingual  Tonsils. — These  are  situated  in  the  base  of 
the  tongue  between  the  circum vallate  papillae  and  the 
epiglottis.  They  are  rounded  masses  of  adenoid  tissue 
composed  of  solitary  follicles  lying  mostly  in  the  tunica 
propria,  and  causing  projections  of  the  surface  that  are 
easily  seen.  In  the  centre  of  each  mass  is  a  deep  depression 
forming  a  blind  pouch,  known  as  the  crypt  (Fig.  256).  This 
is  lined  with  stratified  squamous  epithelium  like  that  of  the 
adjoining  mucous  membrane  except  that  at  various  places 


THE  TONSIL 


333 


the    lymphocytes    have    pushed    their    way   through    the 
epitheUal  cells,  and  escape  on  the  surface. 


Epithelium 
of  pharynx— -p- 


Fig.  257 
Epithelium  of  crypt 


\  Fnllidc         I 


BJood  vessel 


X 


Mucous  glands<' 


fe#f 


'¥ 


V 


Connective-tissue 
capsule 


Section    through    a   dog's   tonsil.      At   x  x    there    are    seen    leukocytes   which    have 
wandered  out  from  the  follicles.     (15  X)      (Szymonowicz.) 


334  THE  MOUTH  CAVITY 

The  Palatine  Tonsils.— These  lie  at  the  base  of  the  tongue 
between  the  anterior  and  the  posterior  pillars  of  the  pharynx. 
They  are  much  larger  than  the  lingual  tonsils  and  are  com- 
posed of  from  ten  to  twenty  follicles  and  a  number  of  crypts. 
The  epithelium  covering  them  is  pierced  in  many  places  by 
encroachments  of  the  adenoid  tissue.  The  crypts  always 
contain  many  lymphocytes  (Fig.  257).  These  are  what  are 
ordinarily  called  the  tonsils,  the  infection  of  which  produces 
tonsillitis. 

The  Pharyngeal  Tonsils. — These  lie  on  the  posterior  wall 
of  the  nasal  phar^^nx  above  the  level  of  the  palate.  Their 
structure  is  similar  to  that  of  the  palatine  tonsil.  The 
crypts  are  five  to  six  in  number  and  are  often  clothed  with 
ciliated  epithelium.  Into  them  open  the  ducts  of  mixed 
glands  which  form  a  distinct  layer  under  the  follicle.  Here 
also  there  is  a  migration  of  lymphocytes  through  the  epi- 
thelium. It  is  the  hypertrophy  of  these  which  form  the 
adenoids  so  often  found  in  children. 


CHAPTER  XXV 

BIOLOGICAL   CONSIDERATIONS   FUNDAMENTAL   TO 
EMBRYOLOGY 

History.— Before  beginning  the  study  of  embryology 
some  topics  in  general  histology  must  be  reviewed,  and  some 
general  biologic  ideas  considered.  No  real  conception  of 
the  complicated  processes  of  individual  development  can  be 
obtained  without  laying  a  foundation  in  the  study  of  the 
cell  as  the  units  of  life  and  the  mechanism  through  which 
the  phenomena  of  life  are  manifested. 

In  embryology  it  is  found  that  the  individual  in  his  physical 
development  passes  through  stages  which  correspond  to 
the  development  of  the  race  or  species  to  which  he  belongs, 
and  a  like  comparison  might  be  drawn  in  mental  develop- 
ment and  the  acquirement  of  knowledge.  This  is  specially 
true  of  the  subject  of  embryology. 

Apparently  the  first  ideas  to  occupy  the  speculative 
thought  of  man  when  he  became  conscious  of  himself  as  an 
independent  being  were  the  questions  of  his  origin  and  the 
relation  to  his  environment  and  destiny.  These  have  become 
the  basis  for  the  development  of  all  religious  thought. 

Up  to  the  beginning  of  the  nineteenth  century  all  con- 
siderations of  these  subjects  were  purely  speculative.  The 
old  question  of  "What  is  life?"  received  endless  discussion. 
In  the  nineteenth  century  this  question  has  been  dropped  into 
the  background,  and  the  question,  "What  is  the  mechanism 
of  life?"  has  been  substituted  for  it.  The  consideration 
of  the  latter  question  has  resulted  not  only  in  the  mar- 
vellous advancement  of  medical  knowledge  and  suigical 
skill,  but  in  the  great  development  of  deeper  fundamental 
thoughts.  It  must  not  be  forgotten,  however,  that  the 
development  of  knowledge  resulting  from  the  considera- 


?)?)C)     BIOLOGICAL    CONSIDERATIONS   OF    EMBRYOLOGY 

tion  of  the  latter  question  has  not  and  does  not  promise  to 
answer  the  old  question,  ''What  is  life?"  any  more  than  the 
laws  of  electricity  and  their  application  to  its  use  answer 
the  question,  "What  is  electricity?" 

The  discovery  of  the  cell  hypothesis  and  the  propounding 
of  the  theory  of  organic  evolution  have  been  the  greatest 
factors  in  the  unification  of  knowledge  and  the  stimulation 
of  thought  in  these  fields.  It  is  interesting  to  notice  that 
these  two  theories,  closely  related  as  they  have  become,  had 
entirely  independent  origins  and  were  long  followed  out  with- 
out any  immediate  connection.  The  theory  of  evolution  w^as 
based  upon  consideration  of  the  forms  of  living  things,  their 
distribution  and  adaptation  to  environment. 

The  Cell  Theory. — The  cell  theory  had  its  origin  in  the  study 
of  miiuite  forms.  Its  beginnings  were  made  possible  by  the 
development  of  the  compound  microscope,  which  revealed 
their  structure  and  showed  them  to  be  small  bodies  made  up 
of  apparently  a  structureless,  granular  material  which  was 
called  protoplasm,  or  the  ultimate  substance  of  life.  This 
material,  as  its  name  indicates,  was  originally  supposed  to 
be  simple  in  structure  and  composition  and  to  be  the  life 
substance.  Huxley's  characterization  of  it  as  the  "physical 
basis  of  life"  w^as  the  beginning  of  the  study  w^hich  has 
revealed  it  to  be  very  far  from  a  simple  substance,  but  rather 
extremely  complex  both  in  structural  arrangement  and 
chemical  composition.  In  more  recent  biology,  therefore, 
the  word  protoplasm  is  being  dropped  and  the  word  cyto- 
plasm or  cell  substance  substituted  for  it. 

The  early  history  of  the  cell  theory  was  obstructed  in 
its  development  by  the  remains  of  the  old  Greek  idea  that 
living  things  could  originate  from  non-living  matter,  that  the 
swamp  breeds  disease,  and  the  decomposing  body  of  an 
animal  bred  maggots.  It  required  fifty  years  of  work  on 
the  cell  theory  for  Virchow,  in  1850,  to  propound  his  thesis 
that  all  living  cells  are  derived  from  a  preexisting  cell,  and 
so  establish  the  continuity  of  life,  which  has  flowed  on  from 
the  beginning  in  an  uninterrupted  stream,  each  individual 
being  only  a  period. 


CELL  DIVISION  337 

When  Schwank  and  Schleiden  showed  that  the  bodies  of 
both  plants  and  animals,  instead  of  being  made  up  of  homo- 
geneous tissue,  were  composed  of  millions  of  structural 
elements  which  they  called  cells,  the  consideration  of  both 
plants  and  animals  were  for  the  first  time  put  upon  a  common 
basis.  Naturally  enough  the  first  thing  to  attract  attention 
was  the  study  of  the  form  and  arrangement  of  these  struc- 
tural elements  in  the  tissues  of  animals  and  plants 

In  following  out  this  study  it  became  more  and  more 
evident  that,  while  infinitely  varied  in  the  detail  of  their 
form  and  structure,  all  cells  had  a  common  plan  of  organi- 
zation and  possessed  structural  characteristics  common  to 
all,  at  least  in  some  stages  of  their  history. 

Relation  of  the  Nucleus  to  the  Protoplasm. — The  first  point 
to  be  discovered  in  the  internal  organization  of  the  cell  was 
the  nucleus,  the  meaning  of  which  and  its  relation  to  the 
cytoplasm  at  once  attracted  attention.  As  the  result  of  a 
vast  amount  of  work,  it  was  gradually  established  that  the 
nucleus  "exerts  a  controlling  and  directing  influence  over 
the  activity  of  the  cytoplasm;"  that  a  cell  deprived  of  its 
nucleus  would  continue  to  live  for  a  longer  or  shorter  time, 
but  that  it  would  not  grow  and  would  not  reproduce  another 
cell;  that  the  phenomena  of  life  manifested  by  destructive 
metabolism  would  continue  until  the  identity  of  the  cyto- 
plasm was  destroyed,  but  there  would  be  no  constructive 
metabolism.  The  work  of  the  cytoplasm  is,  therefore, 
dependent  upon  the  character  of  the  nuclear  material. 

Cell  Division. — As  first  observed,  cell  division  was  supposed 
to  be  an  irregular  cutting  of  the  cytoplasm  and  the  nucleus 
in  two,  forming  two  individual  cells.  The  cytoplasm  by  its 
constructive  changes  does  not  continue  to  increase  indefi- 
nitely, but  as  soon  as  a  certain  size  is  reached  it  divides,  a 
portion  of  the  nucleus  going  to  each  of  the  parts,  which 
immediately  begin  to  increase  in  size.  It  was  soon  found 
that  cell  division  was  not  always  so  simple,  and  that  in  some 
cases  changes  in  the  nucleus  preceded  the  division  of  the 
cytoplasm.  Two  forms  of  cell  division  are  therefore  de- 
scribed, the  simple  or  direct,  and  indirect  or  karyokinetic 


338     BIOLOGICAL    CONSIDERATIONS  OF   EMBRYOLOGY 

t'oll  di\'ision.  The  simple  is  now  known  to  l)e  comparatively 
rare. 

Indirect  Cell  Division. — Indirect  cell  division  must  be  con- 
sidered as  a  means  by  which  the  chromatic  material  of  the 
nucleus  is  equally  and  systematically  distributed  to  the 
resulting  cells.  The  nucleus,  in  cell  division,  contains  a 
beautiful  structural  mechanism,  by  which  the  material 
which  is  to  control  the  development  of  the  resulting  cells 
and  their  activity  is  definitely  distributed  to  them.  In  this 
process  there  is  no  irregularity  in  the  kind  or  amount  of 
material  given  to  the  two  cells. 

In  this  process  the  chromatin  of  the  original  nucleus  is 
divided  into  a  definite  number  of  pieces  which  are  split  in 
two,  and  half  of  each  sent  to  each  new  nucleus,  where  they 
form  its  chromatin  network. 

The  Vehicle  of  Transmission. — It  was  discovered  that  the 
number  of  chromosomes  was  constant  in  every  cell  division 
for  all  the  cells  of  all  the  tissues  of  the  given  species,  and  was, 
therefore,  a  characteristic  of  the  species;  and  that  in  all  the 
cells  of  the  body  it  was  always  an  even  number,  and  that 
in  the  germ  cells  of  the  species  the  number  of  chromosomes 
was  exactly  half  that  in  the  cells  of  the  body.  This  led  to 
the  immediate  recognition  of  the  chromatic  material  as 
the  vehicle  of  transmission.  When  in  the  study  of  fertiliza- 
tion it  was  found  that  fertilization  consists  in  the  union 
of  two  cells,  each  contributing  both  cytoplasm  and  nucleus, 
and  that  the  amount  of  chromatic  material  was  equal  from 
each,  and  exactly  half  that  found  in  the  cells  of  the  parent 
body,  the  equality  of  the  sexes  in  transmission  was  firmly 
established  upon  a  cytologic  basis.  It  is  interesting  to  note 
that  this  equality  had  previously  been  claimed  by  the  dis- 
ciples of  the  evolutionary  theory,  and  it  was  in  this  field 
that  the  evolutionary  theory  and  the  cell  theory  first  met 
on  common  grounds  (about  1875). 

All  the  advancement  in  modern  thought  concerning  hered- 
ity and  transmission  has  resulted  from  these  discoveries. 
The  practical  results  are  perhaps  still  more  important  in 
the  artificial  breeding  of  plants  and  animals,  adapting  them 


CHEMICAL  IDEAS  339 

to  their  environment.  The  work  of  such  men  as  Burbank 
may  be  said  to  be  the  apphcation  of  the  knowledge  of  the 
mechanism  of  cell  division  and  inheritance  to  horticulture 
and  agriculture. 

Chemical  Ideas. — At  the  present  time  the  structural 
mechanism  of  life,  while  inviting  many  fields  for  research, 
may  be  said  to  have  nearly  reached  the  limit  of  possibilities 
of  observation,  and  at  the  present  time  the  chemical  phase 
is  attracting  the  greatest  attention.  Such  questions  as, 
"How  does  the  nucleus  influence  the  activity  of  the  cyto- 
plasm?" are  being  eagerly  investigated.  Cytoplasm,  while 
enormously  complex  in  chemical  composition,  must,  never- 
theless, always  be  thought  of  as  performing  its  vital  func- 
tions by  chemical  activity.  It  is  constantly  building  simpler 
molecules  into  its  own,  and  so  increasing  in  amount.  For 
this  its  surface  must  be  bathed  in  materials  with  which  it 
can  react.  It  is  evident  that  if  the  mass  increased  indefi- 
nitely the  volume  would  increase  much  more  rapidly  than  the 
surface,  and  this  puts  a  limit  upon  the  growth. 

The  constructive  metabolism  of  the  cytoplasm  is  depend- 
ent upon  the  presence  of  the  chromatin  in  the  nucleus.  In 
the  process  of  metabolism,  therefore,  there  must  be  inter- 
action between  the  chemical  substances  of  the  chromatin, 
cytoplasm,  and  food  material.  The  development  of  physio- 
logic chemistry  is  rapidly  affecting  the  ideas  of  the  cause 
and  treatment  of  disease,  and  especially  the  production  of 
immunity  and  susceptibility. 

If  the  dental  profession  is  to  keep  pace  with  the  develop- 
ment in  these  fields  and  apply  the  results  of  investigation  to 
the  treatment  of  diseases  of  the  mouth,  the  study  of  the 
fundamental  sciences  must  be  more  thorough. 


CHAPTER  XXVI 
EARLY  STAGES   OF  EMBRYOLOGY 

Since  fertilization  consists  essentially  in  the  union  of  the 
chromatin  from  two  cells,  and  as  the  result  of  the  union 
restores  the  normal  amount  of  chromatin  for  the  cells  of 
that  species,  it  is  evident  that  in  some  way  the  germ  cells 
must  be  prepared  for  fertihzation  by  the  loss  of  half  their 
chromatin.  This  process  was  first  observed  in  case  of  the 
ovum. 

Maturation. — In  observing  fertilization  of  eggs  of  the  star- 
fish and  various  threadworms,  it  was  noticed  that  before 
fertilization  occurred  the  nucleus  of  the  ovum  divided  with 
karyokinetic  figures,  forming  three  small  bodies  known  as 
polar  bodies.  This  process  is  diagrammed  in  Fig.  258.  In 
reality,  the  ovum  first  divides,  forming  one  polar  body;  the 
polar  body  and  the  ovum  both  then  divide  again,  so  that  the 
result  of  the  two  series  of  division  is  the  formation  of  four 
cells,  one  of  which  is  functional,  three  disappearing.  This 
process  is  practically  universal  in  the  formation  of  ova  of 
both  plants  and  animals.  The  cells  in  the  ovary  which  form 
the  ova  are  called  oogonia.  The  cells  formed  from  these 
are  the  primary  oocyte.  The  division  of  this  cell  produces 
two  secondary  oocytes,  of  which  one  disappears  later.  The 
division  of  the  secondary  oocyte  results  in  the  ovum  and 
three  polar  bodies.  The  number  of  chromosomes  in  the 
primary  oocyte  is  half  the  number  characteristic  of  the 
somatic  cells,  but  they  are  made  up  of  four  pieces.  In  the 
secondary  oocytes  they  are  the  same  number  but  double.  In 
the  ovum  and  polar  bodies  they  are  the  same  in  number  and 
single. 

Spermatogenesis. — Exactly  the  same  series  of  changes 
occur   in   the   formation   of  the   spermatozoa.     They   are 


SPERM  A  TOGENESIS 


341 


illustrated  in  Fig.  259.  On  the  outer  wall  of  the  seminiferous 
tubules  are  two  forms  of  cells,  the  spermatogonia  and  the 
cells  of  Sertoli  (Fig.  260) .    The  cell  of  Sertoli  increases  in  size 


Fig.  258 


Diagram  illustrating  the  reduction  of  the  chromosomes  during  the  maturation  ot 
the  ovum:  o,  ovum;  oc^,  oocyte  of  the  first  generation;  oc-,  oocyte  of  the  second 
generation;  P  p,  polar  bodies.     (McMurrich.) 


and  spreads  out  against  the  basement  membrane,  pushing 
the  spermatogonia  away  from  it.  They  now  divide,  forming 
two  cells,  one  of  which  returns  to  the  basement  membrane 


342 


EARLY  STAGES  OF  EMBRYOLOGY 


and  remains  as  the  spermatogonia,  the  other  becomes  a 
primary  spermatocyte.  The  primary  spermatocytes  divide, 
forming  a  secondary;  the  secondary  divide,  forming  sperma- 
tids, which   develop   directly  into  spermatozoa.     By  com- 


FiG    259 


jr^ 


Diagram  illustrating  the  reduction  of  the  chromosomes  during  spermatogenesis: 
sci,  spermatocyte  of  the  first  order;  sc^,  spermatocyte  of  the  second  order;  sp, 
spermatid.     (McMurrich.) 


paring  the  diagrams  they  will  be  seen  to  correspond  exactly 
with  the  formation  of  the  ova,  except  that  all  of  the  cells 
are  small  and  motile.  The  nuclear  changes  also  correspond 
to  those  of  the  ova,  the  primary  spermatocyte  having 
half  the  number  of  tetrad  chromosomes,  the  secondary  half 


FERTILIZATIOX 


343 


the  number  of  diad,  and  the  spermatids  half  the  number  of 
monad  chromosomes. 

Fertilization. — Fertilization  is  essentially  the  same  in  the 
sexual  reproduction  of  all  plants  and  animals.  It  may  be 
easily  observed  in  the  transparent  cells  of  such  animals  as 
the  starfish  and  the  threadworm.  The  spermatozoon  enters 
the  cytoplasm  of  the  ova,  where  it  immediately  loses  its 
characteristic  form  and  develops  into  a  typical  nucleus  (Fig. 
261).     The  ovum  now  has  two  nuclei,  one  of  which  is  called 

Fig.  260 


Diagram  showing  stages  of  spermatogenesis  as  seen  in  diflferent  sections  of  a 
seminiferous  tubule  of  a  rat:  s,  Sertoli  cell;  sc^,  spermatocyte  of  the  first  order; 
sc^,  spermatocyte  of  the  second  order;  sg,  spermatogone;  sp,  spermatid;  sz,  sperma- 
tozoon.    (Von  Lenhossek's  diagram,  from  McMurrich.) 


the  male  pronucleus,  the  other  the  female  pronucleus.  These 
both  form  chromosomes,  the  number  from  each  being  half 
the  number  typical  of  the  species.  These  are  arranged  as 
usual  between  the  centrosomes.  They  divide  longitudinally, 
each  forming  two,  one  of  which  passes  to  either  centrosome, 
where  a  new  nucleus  is  formed,  and  in  the  meantime  the 
cytoplasm  has  divided  so  that  two  cells  are  formed.  The 
nuclear  material  of  these  two  cells  has,  therefore,  been 
equally  derived  from  the  two  parents,  and  it  is  to  control 
all  of  the  future  development  of  the  individual. 


Fig.  261 


IIOLOBLASTIC  SEGMENTATION 


345 


SEGMENTATION 

Holoblastic  Segmentation. — An  idea  of  the  development  of 
the  embryo  can  perhaps  best  be  obtained  by  following  the 
development  of  the  frog.  The  frog's  eggs  are  large  and 
easily  observed,  and  they  contain  only  a  small  amount  of 
yolk  or  food  material,  which  does  not  obstruct  the  observa- 
tion. The  spherical  ovum  first  divides  into  hemispheres ;  these 


Holoblastic  segmentation.    Segmentation  of  frog  diagrammatically  represented. 

two  cells  are  divided  into  four  in  a  plane  at  right  angles, 
and  the  four  are  divided  into  eight  by  a  plane  at  right  angles 
to  the  previous  plane.  This  is  best  understood  by  examining 
the  illustration  (Fig.  262). 


Legend  for  Fig.  261 

Fertilization  of  the  egg  of  Ascaris  megalocephala,  var.  bivale?is.  (Boveri.)  A,  the 
spermatozoon  has  entered  the  egg;  its  nucleus  is  shown  at  (f ;  beside  it  lies  the 
granular  mass  of  "archoplasm"  (attraction  sphere);  above  are  the  closing  phases  in 
the  formation  of  the  second  polar  body  (two  chromosomes  in  each  nucleus).  B,  germ 
nuclei  ($,  J)  in  the  reticular  stage;  the  attraction  sphere  (a)  contains  the  dividing 
centrosome.  C,  chromosomes  forming  in  the  germ  nuclei;  the  centrosome  divided. 
D,  each  germ  nucleus  resolved  into  chromosomes;  attraction  sphere  (o)  double. 
E;  mitotic  figure  forming  for  the  first  cleavage;  the  chromosomes  (c)  already  split. 
F,  first  cleavage  in  progress,  showing  divergence  of  the  daughter  chromosomes 
toward  the  spindle  poles  (only  three  chromosomes  shown)      (Wilson.) 


346 


EARLY  STAGES  OF  EMBRYOLOCY 


The  lines  of  cell  division  proceed  in  a  regular  way,  the 
planes  passing  in  such  direction  as  to  multiply  the  number 
of  cells  by  two  in  each  set  of  divisions.    Very  soon  the  cells 


Fig.  263 


Four  stages  in  the  development  of  amphioxus,  illustrating  the  formation  of  the 
gastrula.  I.  The  blastula,  a  hollow  sphere  of  ceils;  those  at  the  lower  pole  larger 
than  those  at  the  upper  and  filled  with  yolk  granules.  II  Invagination  of  the 
lower  pole,  because  of  more  rapid  growth  of  cells  at  the  upper  pole.  III.  The 
gastrula,  complete  invagination;  the  creature  is  now  a  two-layered  bag.  A  space 
should  be  shown  between  the  layers:  bl,  the  mouth  of  the  bag,  or  blastopore; 
hy,  inner  layer  of  cells — -hypoblast;  ep,  outer  layer  of  cells — epiblast.  IV.  The 
gastrula  will  now  e'ongate;  the  cavity  becomes  the  alimentary  canal;  the  blastopore 
the  orifice  at  one  end. 


HYPOBLAST  347 

around  the  black  pole  show  a  tendency  to  divide  more 
rapidly  than  those  at  the  white  pole.  At  this  stage  the 
individual  is  made  up  of  a  hollow  sphere  of  cells  with  a 
space  at  the  centre,  the  cells  at  the  upper  surface  being 
small  and  rapidly  dividing,  those  at  the  lower  surface  large 
and  slowly  dividing  (Fig.  263).  As  this  continues  the  sphere 
becomes  flattened  on  the  bottom,  and  finall}'  the  lower  surface 
is  turned  inward  until  the  sphere  is  converted  into  a  hollow 
bag  or  sac  made  up  of  two  layers  of  cells,  the  outer  of  which 
are  small,  the  inner  large,  the  two  joining  around  the  mouth 
of  the  sac.  This  hollow  bag  stage  is  known  as  the  gastrula. 
The  cavity  of  the  sac  is  really  a  part  of  the  outside  world 
around  which  the  cells  have  grown,  and  will  form  the  cavity 
of  the  alimentary  canal.  The  opening  of  the  sac  is  known 
as  the  blastopore,  and  will  form  the  anterior  opening  into 
the  aHmentary  tract  from  the  mouth  cavity.  At  this  stage 
the  individual  is  made  up  of  two  kinds  of  cells,  and  is  to  be 
compared  in  structure  with  the  celenterates  or  such  animals 
as  the  fresh  water  hydra  and  the  coral  polyp. 

Formation  of  the  Germ  Layers. — The  cells  which  form  the 
outer  layer  of  the  gastrula  are  called  the  epiblast,  the  cells 
which  line  it  the  hypoblast  or  entoblast.  Where  these  two 
layers  join  around  the  opening  of  the  blastopore,  a  ring  of 
cells  is  formed  which  differs  from  both  in  form  and  arrange- 
ment, and  will  form  the  mesoblast.  In  the  process  of  cell 
division  from  the  ovum,  therefore,  three  kinds  of  cells  have 
resulted  which  represent  the  first  stage  of  specialization. 

Epiblast. — From  the  cells  of  the  epiblast  will  be  formed: 
(1)  The  epithelium  of  the  surface  of  the  body  and  all  glands 
that  connect  with  it,  the  hair,  the  nails,  and  the  enamel  of 
the  teeth;  (2)  the  epithelium  lining  the  mouth  and  the  nose 
cavities  and  the  lower  part  of  the  rectum;  (3)  the  nervous 
system  and  all  of  the  organs  of  special  sense. 

Hjrpoblast. — From  the  hypoblast  will  be  formed:  (1)  The 
epithelium  lining  the  alimentary  canal  and  the  glands 
that  open  from  it;  (2)  the  epithelium  lining  the  larynx, 
trachea,  and  the  lungs;  (3)  the  epithelium  of  the  bladder 
and  ureter. 


348  EARLY  STAGES  OF  EMBHYOLOGY 

Mesoblast. — From  the  mesoblast  will  be  formed:  (1)  The 
various  connective  tissues,  including  bone,  dentine,  and 
cementum;  (2)  the  muscles,  both  striated  and  unstriated; 
(3)  the  circulatory  system,  including  the  blood  itself  and 
the  lymphatics;  (4)  the  lining  membrane  of  the  serous 
cavities  of  the  body;  (5)  the  kidney;  (6)  the  internal  organs 
of  reproduction. 

Looking  at  these  germ  layers  in  another  way,  it  may  be 
said  that  through  the  mechanism  of  cell  division  all  of  the 
chromatin  which  is  to  control  nerve  cytoplasm  has  been 
distributed  to  the  epiblast;  all  that  which  is  to  contribute 
the  muscular  activit}^  to  the  mesoblast,  and  so  on. 

Meroblastic  Segmentation.  —  If  the  development  of  the 
chick  is  compared  to  that  of  the  frog  they  at  first  seem 
to  be  very  different.  The  ova  of  birds  and  reptiles  are 
provided  with  a  vast  amount  of  food  material  or  yolk, 
which  is  provided  by  the  parent  for  the  nourishment  of 
the  embryo.  It  has  been  seen  that  the  frog's  egg  con- 
tains a  certain  amount  of  yolk,  and  that  the  presence  of 
yolk  granules  retarded  the  cell  division.  In  the  case  of  the 
birds  and  reptiles  the  yolk  granules  have  increased  until 
the  active  cytoplasm  is  left  as  a  small  disk  floating  on  top 
of  a  sphere  of  yolk  enclosed  in  the  yolk  membrane.  The 
white  spot  seen  floating  on  the  top  of  the  yolk  of  a  hen's 
egg  is  called  the  germinal  spot.  Before  fertilization  this  is 
a  mass  of  protoplasm  with  a  nucleus  in  the  centre.  When 
segmentation  begins  it  divides  first  into  right  and  left  halves, 
then  divides  again  by  a  line  at  right  angles  to  the  first  one, 
then  the  four  cells  are  converted  into  eight  cells,  as  if  by  a 
circle,  and  the  process  continues  in  this  way  (Fig.  264).  It 
is  best  understood  from  the  diagram.  This  type  of  segmen- 
tation is  known  as  meroblastic,  while  that  of  the  frog  is 
holoblastic. 

Mammalian  Segmentation. — The  mammalian  ova  contain 
very  little  yolk,  as  the  nourishment  of  the  embryo  is  pro- 
vided for  in  an  entirely  different  way.  The  segmentation 
is  holoblastic  (Fig.  265),  but  shows  marked  differences  from 
that  of  the  frog,  and  characteristics  similar  to  those  of  the 


MAMMALIAN  SEGMENTATION 


349 


birds  and  reptiles,  and  this  has  been  an  added  Hnk  to  the 
evidence  of  the  evokitionists,  that  the  mammaha  have  been 
derived  in  evolution  from  the  reptiles. 


Fig.  264 


Segmentation  of  hen's  egg      IMeroblastic  segmentation. 


First  five  stages  of  segmentation  (rabbit's  ovum),  o,  b,  c,  d,  and  e  In  a,  b,  and  c 
the  epiblast  cells  are  larger  than  the  hj'poblastic  ones.  In  e  the  epiblast  cells  have 
become  smaller  and  more  numerous  than  the  hypoblasts,  and  the  epiblastic  spheres 
are  beginning  to  surround  and  close  in  the  hypoblast  cells:  z.p,  zona  pellucida; 
p.gl,  polar  globules;  u,  first  epiblast  cell;   I,  first  hypoblast  cell. 


350 


EARLY  STAGES  OF  EMBRYOLOGY 


After  the  first  few  divisions  the  cells  of  the  upper  pole 
divide  much  more  rapidly  than  those  of  the  lower,  and  grow 


Fig.  2GG 


Sections  of  the  ovum  of  a  rabbit  during  the  later  stages  of  segmentation,  showing 
the  formation  of  the  blastodermic  vesicle:  a,  gastrula  stages;  ent,  hypoblast,  en- 
closed by  ep,  epiblast;  b,  fluid  is  beginning  to  collect  and  separate  the  epiblast 
and  hypoblast;  c,  the  fluid  has  greatly  increased  in  amount,  the  hypoblastic  cells 
adhering  to  the  upper  surface;  d,  the  blastodermic  vesicle;  ect,  the  outer  layer,  epi- 
blast; ent,  hypoblast,  the  inner  layer  adhering  to  the  inner  surface  of  the  epiblast 
at  the  upper  surface,  forming  the  opaque  area. 


down  over  the  others,  enclosing  them.    When  the  large  cells 
have  been  entirely  covered  in  by  the  small  ones,  the  small 


Pig,  267 


^/.};-v@:5 


^W'..: 


~y, 


5^^:|ifrr;r 


-<'o:^. 


m^^M^ 


hz/. 


m.^?T 


'k^^iiM& 


iz.cJuc.  I . . 


A  series  of  sections  through  the  neurenteric  and  notochordal  canai  of  a  mole 
embryo:  p.gr,  tlie  primitive  groove:  ep  epiblast;  me,  mesoblast:  hy,  hypoblast; 
m  yr,    meiluUary   groove       (Heap  J 


352  EARLY  STAGES  OF  EMBRYOLOGY 

ones  contiinie  to  multiply  more  rapidly  and  fluid  collects 
inside  the  sphere,  leaving  the  large  cells  adhering  to  the  inner 
surface  of  the  small  cell  layer  at  one  pole  of  the  sphere  (Fig. 
266) .  At  the  upper  pole  where  the  sphere  is  made  up  of  two 
layers  of  cells  there  is  an  opaque  spot,  or  the  "area  pellu- 
cida,"  from  only  part  of  which  the  embryo  is  developed,  the 
rest  forming  organs  to  provide  it  with  nourishment  during 
the  embryonal  condition. 

Starting  from  the  centre  of  the  opaque  area  on  the  upper 
surface  of  the  sphere  or  blastula,  there  appears  a  streak 
known  as  the  primitive  streak,  caused  by  the  appearance 
of  a  rod  of  cells  lying  between  the  two  layers,  and  from  the 
side  of  this  rod  or  notochord  a  third  kind  of  cell,  different 
from  either  the  large  or  small  cell  layer,  is  formed.  These 
three  kinds  of  cells  make  up  the  three  layers  of  the  blasto- 
derm and  represent  the  first  step  in  differentiation;  or,  to 
state  it  in  a  different  way,  all  of  the  chromatin  which  (Fig. 
267)  directs  nerve  cell  activity  has  been  sent  to  the  outer 
small  cell  layer,  or  epiderm,  all  of  the  chromatin  which 
directs  muscle  cell  activity,  etc.,  has  been  sent  to  the  new 
cells  of  the  third  layer,  or  mesoderm,  while  the  large  cells 
of  the  inner  layer  or  hypoderm  contain  chromatin  to  direct 
most  of  the  secretory  activities  and  the  formation  of  the 
epithelium  of  the  elementary  canal. 


NERVOUS  SYSTEM 

Formation  of  Neural  Canal. — The  epidermal  cells  of  either 
side  of  the  primitive  streak  grow  rapidly,  forming  two 
ridges  with  a  groove  between  them,  which  grows  deeper 
and  deeper  until  the  ridges  bend  over  and  join,  enclosing  a 
tube  which  is  to  be  the  canal  of  the  spinal  cord  (Fig.  268). 
The  anterior  end  of  this  tube  enlarges  into  three  bulbs 
which  correspond  to  the  ventricles  of  the  brain,  and  as 
they  increase  in  sjze  they  fold  over  ventrally  or  toward  the 
centre  of  the  sphere  until  the  first  and  second  are  at  right 
angles  to  the  original  tubular  part. 


FORMATION  OF  NEURAL   CANAL 


353 


Fig.  268 


Stages  in  the  conversion  of  the  medullary  groove  into  the  neural  canal.  From 
tail  end  of  embrj'O  of  the  cat.  jn.g,  medullary  groove:  n.c,  neural  canal;  ch,  noto- 
chord;  ep,  epiblast;  hy.  hypoblast;  me,  mesoblast;  cce,  celom;  am,  amnion.  (After 
Quain.) 

23 


354  EARLY  STAGES  OF  EMBRYOLOGY 

As  the  outer  layer  forms  the  tul)e  of  the  central  nervous 
system,  the  inner  layer  folds  oft'  a  blind  pouch  from  the 
general  cavity  of  the  sphere  which  is  to  form  the  anterior 
part  of  the  alimentary  canal  (Plate  XVIII).  B}^  this  time 
development  is  complicated  by  the  formation  of  the  embry- 
onal membranes,  the  amnion  and  allantois,  but  we  may  omit 
these  entirely  for  our  purposes. 

The  diagram  from  Quain's  Anatomy  (Figs.  2G9  and  270) 
illustrates  the  condition  just  described,  showing  the  embryo 
in  longitudinal  section,  the  bending  over  of  the  anterior 
end  of  the  neural  canal  to  form  the  mid-  and  forebrain  and 
the  foregut,  or  esophagus,  a  blind  pouch  ending  anteriorly 
under  the  mid-brain  and  posteriorly  opening  into  the  cavity 
of  the  sphere  now  called  the  yolk  sac.    This  pouch  is  lined 


Legend  for  Plate  XVIII. 

Figs.  1  to  5. — Diagrammatic  representations  of  longitudinal  and  cross-sections  or 
hen's  egg  in  various  stages  of  incubation.  They  illustrate  how  the  embryo  is  de- 
veloped out  of  the  area  pellucida,  and  the  yolk  sac,  the  serosa,  and  the  allantois  out 
of  the  extra-embryonal  area  of  the  germ  layers.  The  embryo  is  represented  much 
too  large  in  relation  to  the  yolk  sac.  The  yolk  is  represented  in  yellow  and  the  ento- 
derm in  green,  ectoderm  in  blue,  mesoderm  in  red,  and  the  black  dotted  lines  indicate 
the  limit  to  which  the  inner  and  outer  germ  layers  have  extended  over  the  yolk. 
The  red  dots  mark  the  limit  of  the  mesoderm:  ak,  outer  germ  layer  (blue);  mw, 
medullary  ridges  or  folds;  A",  neural  tube;  am,  amniotic  fold;  vof,  hof,  saf,  anterior, 
posterior,  and  lateral  amniotic  folds;  A.  amnion,  ah,  amniotic  cavity;  S,  serous  mem- 
brane; hu,  dermal  umbilicus;  s/,  lateral  folds;  kf  1,  kf  2,  head  fold;  afb,  ifb,  outer 
and  inner  limb  fold;  iA;,  inner  germ  layer  (green);  rr,  its  margin  of  overgrowth; 
dr,  intestinal  groove;  dg,  vitelline  duct;  al,  allantois;  ds,  interstitial  sac;  du,  intes- 
tinal umbilicus;  mk,  middle  germ  layer  (red);  mk,  parietal  layer  of  mesoderm; 
mk,  visceral  layer  of  mesoderm;  st,  lateral  limits  of  the  same;  dm,  vm,  dorsal  and 
ventral  mesenteries;  th\  bodj'  cavity;  th^,  th^,  embryonic  extra-embryonic  parts  of 
the  same. 

Fig.  1. — Cross-section  through  hen's  egg  on  second  day  of  incubation. 

Fig.  2. — Cross-section  through  hen's  egg  on  third  day  of  incubation. 

Fig.  3. — Longitudinal  section  through  hen's  egg  on  third  day  of  incubation. 

Fig.  4. — Longitudinal  section  through  hen's  egg  beginning  of  fourth  day  of 
incubation. 

Fig.  5. — Longitudinal  section  through  hen's  egg  on  seventh  day  of  incubation. 

Fig.  6. — Cross-section  through  embryo,  first  day. 

Fig.  7. — Diagrammatic  longitudinal  section  through  a  selachian  embryo. 

Fig.  8  (Kolhkie). — Half  of  a  cross-section  through  embrj'o  chick  (two  daj-s). 

Fig    9  (Kollikie). — Cross-section   through  embryo  chick,  beginning  of   third  day. 

Fig.  10. — Cross-section  of  chick  (five  days)  in  the  region  of  the  umbilicus. 

Fig.  11. — Diagrammatic  longitudinal  section  of  embryo  chick. 


PLATE   XVIII 


alf 


BRANCHIAL  ARCHES 


355 


by  hypoblast  and  covered  by  mesoblast  and  epiblast.  The 
heart  has  already  begun  its  development  in  the  mesoblast 
on  the  ventral  side  of  the  foregut. 

Branchial  Arches. — There  now  appear  what  are  called  the 
gill  slits,  openings  from  the  foregut  through  its  walls  to  the 

Fig.  209 


-Allantoic 
Hi  ad  qut 


Diagram  of  a  longitudinal   section  of    a   mammalian   embryo.     Very  earlj-,  showing 
the  folding  off  of  the  embryo.      (After  Quain.) 


surface  of  the  embryo,  which  are  separated  by  thickenings 
of  the  wall  forming  arches  around  the  gut  known  as  the 
visceral  or  branchial  arches,  at  the  centre  of  each  of  which 
is  found  a.  bloodvessel.  These  structures  are  to  be  compared 
to  the  gills  of  a  fish,  which  are  slits  through  the  wall  of  the 


356 


EARLY  STAGES  OF  EMBRYOLOGY 


esophagus  to  the   outside,  so  that  water  taken  into  the 
mouth  nia}'  pass  out  through  the  slits.    At  this  time,  too. 


Fig.  270 


Median  sections  through  the  head  of  embryo  rabbits  five  (.4.)  and  six  (B)  milli- 
rueters  long:  A,  the  opening  from  the  foregut  has  not  yet  been  made;  B,  the  faucial 
opening  is  shown  at  /;  c,  first  brain  vesicle;  mc,  midbrain  vesicle;  mo,  medulla 
oblongata;  m,  medullary  epiblast;  i/,  infundibulum;  spe,  sphenothenoidal,  be,  sphe- 
noidal, and  sp.o,  sphenooccipital  parts  of  the  basal  cranii;  i,  foregut;  ch,  notochord; 
py,  buccal  pituitary  involution;  a7n,  amnion;  h,  heart. 


Embryo  showing  branchial  arches  and  stomodeum. 


PLATE   XIX 


Second  aortic  arch. 
TJiird  aortic  arch. 


Auditory  vesicae. 

Primitive 
jugidar  reni.N 

Fourth  aortic  arch 

■    Fifth  aortic, 
arch. 

Dorsal  aorta. 


Cardinal  rein.- 


Mid-gut. 


Hind-gut. 


Umbilical  vein 


First  aortic  arch. 


,  Olfactory  pit. 

^_' 

Maxilla ry  p rocess. 

Hyomandihular  cleft. 

Mandibular  arch. 

Aortic  bidb. 

Auricle. 

Duct  of  Cnrier. 

Ventricle. 

Vitelline  rein 
Yolk  sac. 


Allantois. 

Umbilical  [allantoic  artery). 


Profile  View  of  a  Human  Embryo  Estimated  at  Twenty-one 
Days  Old.     (After  His.) 

Showing   branchial   arelies   and   relation  to   bloodvessels. 


FRONTONASAL  PROCESS  357 

the  arrangement  of  the  bloodvessels  exactly  resembles  that 
of  a  fish,  and  tl^e  individual  may  be  said  to  be  in  the  fish 
stage  of  development. 

Stomodeum. — Plate  XIX,  from  Quain's  Anatomy,  and  Fig. 
271,  from  Hertwig's  Text-Book  of  Embryology,  shows  the  em- 
bryo at  this  stage  and  the  arrangement  of  the  bloodvessels. 
As  the  forebrain  grows  ventrally,  the  first  visceral  arch,  or 
mandibular  arch,  also  grows  in  the  same  direction,  and  the 
space  between  the  inferior  surface  of  the  forebrain  and  the 
upper  surface  of  the  first  arch  is  the  beginning  of  the  mouth 
and  nose  cavities,  now  called  the  stomodeum.  From  the 
base  of  the  mandibular  arch  is  seen  also  the  rounded  bud, 
which  is  beginning  to  grow  forward  along  the  base  of  the 
forebrain  to  form  part  of  the  maxillary  arch,  and  finally  the 
upper  jaw.  At  this  time  also  the  area  which  is  to  develop 
the  sense  of  smell  appears  on  each  side  at  the  outer  and 
lower  portion  of  the  forebrain.  The  olfactory  areas  grow  out 
of  the  base  of  the  forebrain,  at  first  being  on  the  outside  of 
the  head  and  in  the  later  development  being  enclosed, 
leaving  an  opening  to  the  surface — the  nostril. 

If  we  have  gained  a  correct  idea  of  the  conditions  just 
described  by  means  of  the  pictures,  it  will  be  understood 
that  by  the  growing  forward  of  the  mandibular  arch  there  is 
left  an  almost  cubical  space  between  the  lower  surface  of 
the  fore-  and  midbrain  and  the  upper  surface  of  the  mandib- 
ular arch  (Fig.  271).  This  is  a  part  of  the  outside  world, 
and  is  enclosed  to  form  the  mouth  and  nose  cavities.  This 
process  is  best  understood  if  we  think  of  the  development 
from  the  anterior  end  of  the  forebrain  of  a  process  which 
may  be  described  as  a  curtain  dropping  down,  making  a 
central  piece,  and  the  bud  from  the  mandibular  arch  on  each 
side  growing  forward  to  unite  with  it,  leaving  a  slit  between 
them  and  the  mandibular  arch  which  will  be  the  mouth.  In 
order  to  get  a  correct  idea  of  this  process  it  must  be  followed 
somewhat  more  minutely. 

Frontonasal  Process. — As  the  frontonasal  process  develops 
it  is  made  up  of  four  rather  bulk-hke  portions  (Figs.  272  and 
273),  two  occupying  the  centre  and  which  develop  into  the 


358 


Olfactorj 
pit 


Lens. 


EARLY  STAGES  OF  EMBRYOLOGY 

Fig.  272 

Maxillanj  process.  • 

Mandibular  arch. 

Ilyo-mandihular  cleft. 


iuditory  vesicle. 

V 


llyoid  arch. 
'^Sj^  Thyro-hyoid  arch. 


1 


Simis 

..^  jjrsecervicalis 


w 


y 


The  beginning  of   the  mandibular  arch  and  the  maxillary  buds 
Fig.  273 
Cerebral  hemisphere. 


Fronto-nasal 
process. 

Stomodaenm. 


Lateral  nasal  process. 

1      -Eye. 
^     "-  Processus  (jlobularis. 
^         Maxillary  process. 

x^  Mandibular  arch. 

^,    11  /o-mandibular  cleft. 


An  embryo  a  little  older  than  Fig.  272.      Viewed  from  in  front.      Showing  develop- 
ment of  maxillary  bud.s  and  frontonasal  process. 


FRONTONASAL  PROCESS 


359 


intermaxillary  bone  containing  the  incisor  teeth  and  the 
centre  of  the  lip;  and  two  side  or  lateral  processes  which 
grow  out  around  the  olfactory  area  and  form  the  alee  of  the 


Fig.  274 


Embryo,  a  little  older  than  Fig.  273.  A.  front  view,  frontonasal  process,  and 
maxillary  buds  about  to  unite:  1,  lateral  nasal  part  of  frontonasal  process;  2, 
maxillary  bud;  3,  mandibular  arch;  4,  hyoid  arch.  B,  the  same  embryo  with  the 
mandibular  arch  removed:  1,  horizontal  growth  of  the  maxillary  bud;  2,  lateral 
nasal  process;  3,  mesial  nasal  process;  4,  globular  processes  which  form  the 
horizontal  part  of   the  intermaxillary  bone. 

Fig    275 


Head  of  an  embryo  of  about  seven  weeks.  (His.)  The  external  nasal  pro- 
cesses have  united  with  the  maxillary  and  globular  processes  to  shut  off  the 
olfactory  pit  from  the  orifice  of   the  mouth. 


360 


EARLY  STAGES  OF  EMBRYOLOGY 


nose  surrounding  the  nostril.  These  do  not  unite  again  with 
the  central  parts,  but  the  end  stops  over  the  point  where  the 
maxillary  bud  unites  with  the  central  process  (P'igs.  274  and 
275).  A  failure  of  union  causes  the  deformity  of  harelip, 
the  opening  in  the  lip  extending  to  one,  or,  if  double,  to  both 
nostrils. 

When  the  central  part  of  the  frontonasal  process  has 
united  with  the  maxillary  bud  on  each  side  the  arch  of  the 
upper  jaw  is  complete  and  the  original  cubical  space  or 


Fig.  27G 


Mouth  of  olfactory 

uit.  (ir  vosfril. 


Palatid  process  of  pro 
cessm  globulnris 


Palatal  part  of  maxil 
lary  process. 

Maxillary 
process 


The  head  of  an  embryo  with  the  mandibular  arch  removed.  Looking  up  from 
the  mouth  into  the  nose  cavity.  The  union  of  the  globular  processes  forming  the 
anterior  part  of  the  palate,  and  the  horizontal  ingrowths  from  the  maxillary  buds, 
showing  the  way  in  which  they  unite  from  before  backward,  separating  the  nose 
from  the  mouth  cavity. 


stomodeum  is  enclosed,  leaving  only  the  slit  between  the 
maxillary  and  mandibulary  arches  which  is  to  form  the 
mouth;  but  the  enclosed  space  is  in  one  chamber,  there 
being  no  separation  between  the  mouth  and  nose  cavities. 
The  time  of  this  development  in  the  human  embryo  may 
be  placed  at  about  the  fourth  week. 

Separation  of  Mouth  and  Nose  Cavity. — The  separation  of 
the  mouth  and  nose  cavities  occurs  by  the  development  of 
horizontal  ingrowths  from  the  three  parts  making  up  the 


SEPARATION  OF  MOUTH  AXD  NOSE  CAVITY     361 

maxilla  and  beginning  at  the  centre  and  progressing  back- 
ward. First,  a  small  triangular  piece  from  the  central  part 
of  globular  processes  of  the  frontonasal  process,  this 
uniting  with  the  horizontal  or  palatal  process  of  the  maxil- 
lary buds  on  each  side  until  these  reach  the  apex  of  the 
triangle,  which  will  be  the  intermaxillary  bone,  just  a  little 
way  back  in  the  palate,  and  from  here  backward  they 
unite  with  their  fellow  of  the  opposite  side.  This  is  best 
seen  by  removing  the  mandibular  arch  and  viewing  the 
parts  from  below  (Fig.  276,  from  Hertwig's  Embryology). 

The  deformity  of  cleft  palate  is  then  a  later  development 
than  that  of  harelip,  and  either  may  occur  without  the  other, 
though  they  are  usually  found  together.  The  cleft  of  the 
palate  usually  turns  to  one  side  at  the  front,  running  out 
between  the  cuspid  and  lateral  unless  it  is  double,  when  a 
detached  piece  is  found  in  the  centre  in  front,  containing  the 
incisors.  As  soon  as  the  mouth  and  nose  cavities  are  sepa- 
rated and  as  fast  as  bone  is  formed  in  the  jaws  most  of  the 
space  is  occupied  by  the  tooth  germs. 


CHAPTER  XXVII 

THE  DEVELOPMENT  OF  THE  TOOTH  GERM 

The  Dental  Ridge. — By  the  middle  of  the  second  month  of 
development  the  arches  of  both  upper  and  lower  jaws  are 
completed,  and  the  palate  has  separated  the  nose  and  mouth 
cavities.  The  first  indication  of  the  development  of  the 
teeth  is  the  multiplication  of  the  cells  of  the  epiblast  in  a 
curved  line  on  the  crest  of  each  arch  in  the  area  which  is 
to  be  occupied  by  the  teeth.  By  this  multiplication  of  cells 
the  epiderm  is  piled  up  in  a  ridge,  projecting  above  the 
surface,  and  at  the  same  time  the  deep  layer  of  the  epiblast 
is  forced  down  into  the  underlying  mesoderm  (Fig.  277). 
This  structure  is  known  as  the  dental  ridge.  In  sections  the 
cells  piled  up  against  the  surface  are  usually  washed  off  more 
or  less  by  the  reagents,  but  the  depression  into  the  mesoderm 
is  shown.  On  the  hngual  surface  of  this  ridge,  in  the  part 
embedded  in  the  mesoderm,  the  cells  of  the  Malpighian 
layer  grow  out  lingually  at  right  angles  to  the  ridge,  forming 
a  continuous  shelf  known  as  the  dental  lamina  (Fig.  278).  It 
is  important  to  remember  that  the  lamina  is  continuous 
along  the  entire  extent  of  the  ridge. 

The  Enamel  Organ. — From  ten  points  on  the  surface  of  the 
lamina  little  buds  of  epiblast  start  and  grow  down  into  the 
mesoderm,  increasing  in  size  and  becoming  bulbous  at  the 
deep  end.  The  bulbous  portion  gradually  becomes  flattened. 
At  this  stage  the  bulb  is  composed  of  an  outer  layer  of  colum- 
nar cells,  continuous  with  the  Malpighian  layer  of  the  ridge 
and  a  central  mass  of  large  polyhedral  cells  (Fig.  179). 
As  the  bud  continues  to  grow  into  the  mesoderm,  the  meso- 
dermic  tissue  below  it  begins  to  condense  and  the  cells  of  the 
upper  portion  of  the  bulb,  growing  more  rapidly,  convert 
the  bulb  into  a  two-layered  bag. 


THE  DENTAL  PAPILLA 


363 


The  Dental  Papillae. — The  cells  in  the  condensed  mesoderm 
multiply  and  grow  up  into  the  cavity  of  this  cap,  forming  the 
beginning  of  the  dental  papillae.  This  stage  is  represented 
in  Figs.  280  and  2S1,  in  which  the  enamel  organ  is  seen  con- 
nected with  the  lamina  by  a  cord  of  epithelial  cells,  and 


Fig.  277 


The  dental  ridge.     A  section  through  the  mandible  of  a  pig  embryo  :)T  r  h^  : 

two  spicules  of   bone  beginning  to  form,  to  the  right  ^leckel's  cartilage. 


made  up  of  an  outer  layer  of  columnar  cells  known  as  the 
outer  tunic,  and  an  inner  layer  of  columnar  cells  lying  next 
to  the  dental  papillae,  known  as  the  inner  tunic.  The  poly- 
hedral cells  between  the  two  layers  fill  the  central  part  of 
the  enamel  organ  and  have  taken  on  peculiar  appearance, 
which  has  given  to  them  the  name  of  the  stellate  reticulum. 


364      THE  DEVELOPMENT  OF  THE  TOOTH  GERM 


The  development  of  the  tooth  germ  now  progresses  until 
the  dental  papilla  has  taken  on  the  typical  form  of  the 
tooth.  The  fully  formed  enamel  organ  for  an  incisor  of  a 
sheep  is  shown  in  Fig.  282.  The  cord  which  connects  the 
outer  tunic  with  the  surface  epithelium  is  not  shown  in  this 
section. 

Fig.  278 


Thedental  ridge  and  denal  lainina. 

The  Tooth  Germ. — The  tooth  germ  is  composed  of  the 
enamel  organ,  made  up  of  the  outer  tunic,  the  inner  tunic, 
and  the  stellate  reticulum,  covering  the  dental  papillae. 
From  the  base  of  the  papillae  fibrous  tissue  develops,  growing 
upward  around  the  entire  tooth  germ  and  enclosing  it  in  a 
definite  wall  or  sac  of  fibrous  tissue.  This  is  known  as  the 
dental  follicle,  or  the  folhcle  wall. 

The  Dental  Follicle.— This  term  has  been  used  to  indicate 
not  simply  the  connective-tissue  wall,  but  all  of  the  structure 


TOOTH  GERMS  OF  THE  PERMANENT  TOOTH     365 

enclosed  in  it.  This  use  of  the  term,  however,  is  confusing, 
and  the  term  should  be  confined  to  the  fibrous  sac.  By  the 
end  of  the  twelfth  week  the  follicle  wall  has  grown  up  so  as 
to  enclose  the  enamel  organ,  and  the  epithelial  cord  which 
has  connected  it  with  the  surface  is  broken. 

Tooth  Germs  of  the  Permanent  Tooth.^ — Before  the  epithelial 
cord  is  broken,  from  some  point  on  the  lingual  surface  of  the 

Fig.  279 


WM 


'.MW^W^ 


-^ams^^ 


A  section  through  the  ni:anliliuh;r  arch:  E,  enamel  organ;  D,  beginning  of  the 
dental  papilla;  B,  bone;  F,  fold  from  the  side  of  the  mandible  to  the  base  of  the 
tongue    covering  the  beginning  of   the  sublingual  gland;    T,  tongue. 


outer  tunic  or  along  the  cord  a  bud  of  epithelial  cells  grows 
out  and  turns  down  into  the  mesoderm,  passing  over  the 
follicle  wall  (Fig.  283).  This  continues  to  grow  downward 
until  it  has  reached  the  position  below  and  to  the  lingual 
of  the  tooth  germ  for  the  temporary  tooth,  where  it  develops 
into  the  enamel  organ  for  the  corresponding  permanent 
tooth.  It  goes  through  the  same  changes  of  form  as  has 
been  seen  in  the  temporary  teeth. 


.366     THE  DEVELOPMENT  OF   THE  TOOTH  GEJiM 

Beginning  of  Calcification. — About  the  sixteenth  week 
the  tooth  germs  of  all  the  temporary  teeth  have  been  com- 
pletely enclosed  in  their  follicles  and  the  enamel  organ  for 

Fig.  280 


The  enamel  organ.      The  outer  tunic  connected  to  the  lamina  by  the  cord;   the  dental 
papilla  growing  up  into  the  cap.      The  spaces  are  skrinkage  spaces. 


the  corresponding  permanent  teeth  have  begun  their  devel- 
opment (Fig.  284).  This  illustration  shows  a  section  through 
the  lower  jaw  of  a  pig,  and  exhibits  the  tooth  germs  for  two 
incisors  at  about  the  stage  of  the  closing  of  the  follicle  walls. 


BEGINNING  OF  CALCIFICATION 


367 


The  buds  for  the  permanent  teeth  are  seen  on  the  Ungual, 
and  the  formation  of  enamel  and  dentine  is  just  beginning 
in  the  temporary  teeth.  Notice  the  remains  of  ^Meckel's 
cartilage,  and  the  extension  of  endomembranous  bone 
formation  which  is  just  beginning  to  form  a  periosteum  on 


Fig.  2S1 


The  enamel  organ,  a  little  older  than  Fig.  280.  It  shows  the  outer  tunic,  the 
inner  tunic,  and  the  stellate  reticulum.  The  dental  papilla  in  the  hollow  of  the 
cap.      The  spaces  are  caused  by  shrinkage. 


its  surface.  The  bone  has  grown  around  ^Meckel's  cartilage 
and  around  the  tooth  gems  on  the  buccal  and  lingual, 
enclosing  them  in  an  open  groove,  which  will  later  be  com- 
pleted and  divided  into  separate  crypts  for  each  tooth. 
Fig.  2S5  is  from  a  similar  specimen  in  the  region  of  a  tem- 


368      THE  DEVELOPMENT  OF   THE   TOOTH  GERM 

porary  molar.  The  dental  papilla  is  taking  on  the  form  of  a 
crown  and  the  formation  of  enamel  and  dentine  is  ready  to 
begin.     The  cells  on  the  outer  layer  of  the  dental  papilla 

Fig.  282 


The  tooth  germ,  from  the  manchlih  ut  a  sheep  The  enamel  organ  shows  the 
outer  tunic,  inner  tunic,  and  stellate  reticulum.  The  dental  papilla  projects  into 
the  enamel  organ.  The  follicle  is  attached  to  the  base  of  the  dental  papilla  and 
surrounds  the  enamel  organ.      The  spicules  of   bone  form  the  crypt  wall. 


have  developed  into  odontoblasts,  forming  a  single  layer  of 
columnar  cells  lying  in  contact  with  the  inner  tunica  of  the 
enamel  organ.     Here  the  formation  of  enamel  and  dentine 


BEGINNING  OF  CALCIFICATION 


369 


begins,  the  dentine  slightly  preceding  the  enamel.  The 
odontoblasts  form  and  calcify  dentine  matrix  from  without 
inward.     The  cells  of  the  inner  tunic  or  ameloblasts  form 


Fig.  283 


The  tooth  germ  showing  the  bud  for  the  permanent  tooth  at  P.  Calcification  is 
just  beginning:  F,  follicle  wall;  D,  dental  papilla;  T,  inner  tunic;  T',  outer  tunic, 
^',  stellate  reticulum;   O,  odontoblasts;   A,  ameloblasts,   B,  bone. 

24 


370     THE  DEVELOPMENT  OF  THE  TOOTH  GERM 

and  calcify  the  enamel  rods  and  cementing  substance,  pro- 
gressing from  within  outward.  The  line  upon  which  the 
odontoblasts  and  ameloblasts  lie  in  contact,  therefore,  will 
become  the  dento-enamel  junction.  The  formation  of 
dentine  and  enamel  begin  at  separate  points,  which  are  at 
first  very  close  together,  but  are  carried  farther  apart  by  the 
growth  of  the  dental  papilla,  until  they  have  progressed 
along  the  dento-enamel  junction  and  unite,  when  the  increase 
in  the  diameter  of  the  dental  papilla  is  stopped.  This,  per- 
haps, will  be  better  understood  by  studying  Figs.  68  to  73. 

Fig.  284 


A  section  through  the  lower  jaw  of   a  pig  embryo,  showing  germs  of  two  incisors. 


First  Permanent  Molar. — The  origin  and  development  of 
the  first  permanent  molar  differs  from  that  of  all  the  other 
permanent  teeth  in  important  respects.  It  is  the  only 
permanent  tooth  whose  enamel  organ  springs  directly 
from  the  dental  lamina  in  the  same  way  as  those  for  the 
temporary  teeth.  It  is  the  only  permanent  tooth  whose 
crown  is  calcified  before  the  individual  is  thrown  upon  its 


ORIGIN  OF  THE  SECOND  AND  THIRD  MOLARS     371 

own  resources  for  the  obtaining  of  nourishment.  Nature 
seems  to  have  taken  special  precautions  in  the  formation 
of  this  most  important  tooth. 

About  the  seventeenth  week,  at  a  point  on  the  dental 
lamina,  posterior  to  the  enamel  organs  of  the  temporary 
teeth,  a  bud  starts  to  grow  down  into  the  mesoderm,  which 

Fto.  285 


Germ  of   a  premolar  from  an  embryo  pig. 


develops  into  the  enamel  organ  for  the  first  molar,  and  by 
the  ninth  month  the  follicle  is  complete  and  calcification  has 
begun. 

The  Origin  of  the  Second  and  Third  Molars — The  enamel 
organ  for  the  second  molar  is  formed  from  a  bud  given  off 
from  the  outer  tunic  of  the  enamel  organ  of  the  first  molar. 


372     THE  DEVELOPMENT  OF  THE  TOOTH  GERM 

Tlie  enamel  organ  for  the  third  molar  is  formed  from  a  bud 
given  ofl'  from  the  outer  tunic  of  the  enamel  organ  of  the 
second,  at  about  the  third  year. 

Chronology.— The  development  of  the  teeth  was  first 
investigated  by  Lagros  and  Magitot  (about  1865).  Since 
that  time  their  work  has  been  repeated  and  verified  by 
several  investigators.  About  ISSO  Dr.  Black  repeated  the 
entire  work  of  ^Magitot,  and  some  of  his  illustrations  were 
used  by  Dr.  Dean  in  his  Translation  of  Magitot  Memoir. 
Magitot's  table,  showing  the  chronology  of  tooth  develop- 
ment, is  given  on  page  378. 


•3b 


<N   if.:\-^. 


CO 

o 

O 

eriods  at  ^ 

the  teeth 

eruptcc 

6th  moi 
10th  moi 
16th  moi 
20th  moi 
Dth  to  the 
inont  h 
24th  moi 
26th  moi 
28th  moi 
30th  moi 

1 

00 

2  5 

2     o 

c3 

o 

From  5  t 

years 
From  12 
years 
1  From  18 
[     years. 

O.                                    CO 

Periods  at 

which  the 

dentine  cap 

first  appears. 

1       

•    § 

:  & 

:  S 

J3      • 

o  .     ■ 

(  6th  month 
( of  fetal  life 

3d  year 
12th  j'car 

Closing  of 
the  follicle 
and  rupture 
of  the  cord. 

M    -5 
.Sort 

B 

05 

20th  week 

1st  year 

After  the  ) 
Gth  year  j 

Period  at  Time  of  the 
which  the  appearance 
dental  bulb      of  the  fol- 

appears.       licular  wall 

- 

o 

?5 

18th  week 

1st  year 

After  the 
6th  year 

-a 
■  S 

17th  week 

1st  year 

f  After  the 
1  5th  year 

Period  at  which 

the  enamel 

organ  first 

appears. 

i 

•fa 

15th  week 
3d  mo.  after  ) 

birth     ; 

3d  year 

'S.  ^ 

C  o 

013 


>-iCC>-iCQi-iCC!.-i'H<M(M 

uopi^uap  AaBJoduiax 


%  c.2'S  _  50  2  =  ~  =  ::.^  -^  ^^ 

C  ~  C  ~   C  3  (D  ccTST!  tn  oo'3'O'O'a 


noi-ji^uap  ^nauBUijajj 


CHAPTER  XXVIII 

THE  RELATION  OF  THE  TEETH  TO  THE  DEVELOPMENT 
OF  THE  FACE 

At  birth  the  jaws  contain  all  of  the  temporary  teeth  and 
the  first  molars  in  a  partially  formed  condition,  and  the 
follicles  for  all  of  the  permanent  teeth  except  the  second  and 
third  molars.  These  very  nearly  fill  the  substance  of  the 
bone.  In  the  growth  of  the  bones  of  the  face  and  the  changes 
that  occur  in  the  transformation  of  the  child  to  the  adult 
face,  the  teeth  play  a  most  important  role. 

Before  considering  this  subject  in  detail  it  is  necessary  to 
recall  in  this  connection  some  things  that  have  already 
been  emphasized. 


RELATION  OF  THE  TEETH  TO  THE  BONE 

In  evolution  the  teeth  originally  had  no  connection  with 
the  bone,  it  being  formed  later  for  their  support.  In  embry- 
ology the  tooth  is  formed  first,  and  the  bone  formed  around 
it.  In  this  way  the  development  of  the  individual  repeats 
evolution.  In  the  study  of  the  bone  it  has  been  emphasized 
that  the  connective  tissues  have  been  specialized  to  meet 
mechanical  conditions,  and  that  both  ontogenetically  and 
phylogenetically  they  are  formed  in  response  to  mechanical 
stimuli.  The  mutations  of  connective  tissue  have  been 
dwelt  upon,  and  especially  the  fact  that  a  bone  as  an  organ 
of  support  always  contains  fibrous  tissue,  and  that  there  is 
a  continual  oscillation  between  formation  and  destruction, 
by  means  of  which  it  is  perfectly  adapted  to  its  mechanical 
environment.  The  transformations  of  bone  in  bone  growth 
have  been  pointed  out,  and  these  will  be  still  more  carefully 


RELATION  OF   THE   TEETH   TO   THE  BONE        375 

studied  in  connection  with  the  growth  of  the  bones  of  the 
face. 

Some  years  ago  the  author  undertook  a  study  of  the 
structure  and  growth  of  the  jaws  and  alveolar  process,  which 
resulted  in  very  important  modifications  of  the  conceptions 
of  the  matter  as  given  by  standard  texts.    Tomes  describes 

Fig.  286 


Tomes'  diagram  of  development  of  mandible  from  infant  to  adult. 


the  process  of  development  as  essentially  an  addition  at 
the  posterior  portions  of  the  jaws  to  make  room  for  the 
successively  developed  permanent  molars,  and  illustrates 
the  process  in  diagrams  (Fig.  286).^  The  following  quotation 
states  his  view: 

"But  the  main  increase  in  the  size  of  the  jaw  has  been  in 
the  direction  of  backward  elongation;  in  this,  as  Kolliker 
first  pointed   out,   the  thick  articular  cartilage  plays  an 

1  Tomes'  Dental  Anatomj-,  p.  208. 


376      THE   TEETH   AND    DEVELOPMENT  OF    THE   FACE 

important  part.  The  manner  in  which  the  jaw  is  formed 
might  also  be  described  as  wasteful;  a  very  large  amount  of 
bone  is  formed  which  is  subsequently,  at  no  distant  date, 
removed  again  by  absorption;  or  we  might  compare  it  to  a 
modelling  process,  in  which  thick,  comparatively  shapeless 
masses  are  dabbed  on  to  be  trimmed  and  pared  down  into 
form. 

"To  bring  it  more  clearly  home  to  the  student's  mind,  if  all 
the  bone  ever  formed  were  to  remain,  the  coronoid  process 
would  extend  from  the  condyle  to  the  region  of  the  first 
bicuspid,  and  all  the  teeth  behind  that  would  be  buried  in 
its  base;  there  would  be  no  neck  beneath  the  condyle,  but 
the  internal  oblique  line  would  be  a  thick  bar  corresponding 
in  width  with  the  condyle.  It  is  necessary  to  fully  realize 
that  the  articular  surface  with  its  cartilage  has  successively 
occupied  every  spot  along  this  line;  and  as  it  progresses 
backward  by  the  deposition  of  fresh  bone  in  its  cartilage,  it 
had  been  followed  up  by  the  process  of  absorption,  removing 
all  that  was  redundant." 

In  a  similar  way  in  any  maxilla,  the  temporary  dentition 
is  shown  to  occupy  about  the  same  space  as  the  permanent 
teeth,  as  far  as  the  second  bicuspid,  and  the  adult  is  supposed 
to  be  formed  from  the  child  by  the  building  on  of  the  bone 
at  the  back  as  the  molars  are  formed. 

This  conception  is  fundamentally  misleading,  for  if  the 
infant  mandible  were  to  be  shown  in  the  relation  to  that  of 
the  adult  in  three  dimensions  of  space,  it  would  be  found 
to  be  above  and  entirely  within  the  adult  mandible,  and  no 
part  of  the  bone  which  constituted  the  infant  jaw  is  present 
in  the  adult.  In  the  upper,  if  the  temporary  teeth  at  two 
years  were  figured  in  relation  to  those  of  the  adult  they 
would  be  placed  somewhere  up  in  the  nasal  cavity. 

The  conditions  are  more  correctly  stated  by  saying  that 
forces  exerted  at  the  posterior  portions  of  the  jaw  through 
the  development  of  the  successive  molars  cause  the  bone 
to  grow  downward,  forward,  and  outward  in  the  upper  arch, 
upward,  forward,  and  outward  in  the  lower,  carrying  the 
bone  into  an  entirely  new  position  in  space. 


RELATION  OF   THE   TEETH   TO   THE  BONE        377 

In  this  process  the  peridental  membrane,  periosteum, 
and  articular  cartilage  all  play  their  part,  but  all  the  bone 
posterior  to  the  second  bicuspid  cannot  be  thought  of  as 
having  been  formed  by  the  articular  cartilage  and  modelled 
into  form  by  the  periosteum,  as  might  be  inferred  from 
Tomes'  statement. 

Structure  of  Maxillae  and  Mandible. — Before  attempting 
to  follow  the  growth  of  the  bone  in  the  development  of  the 
face,  the  arrangement  and  distribution  of  the  varieties  of 
bone  in  the  structure  of  the  mandible  and  maxillse  should 
be  carefully  studied. 

Cortical  Plate. — The  outer  surface  of  these  bones  is  formed 
of  a  compact  layer  composed  partly  of  subperiosteal  and 
partly  of  Haversian  system  bone.  This  varies  greatly  in 
thickness,  depending  upon  the  stress  to  be  sustained.  It  is 
called  the  cortical  plate. 

Cancellous  Bone. — The  centre  of  the  bone  is  cancellous 
in  character  and  made  up  of  thin  plates  of  lamellae  arranged 
around  large  medullary  spaces.  The  direction  and  arrange- 
ment of  these  plates  is  determined  by  the  forces  received  on 
the  cortical  plates  and  the  directions  of  stress  to  which  they 
are  subjected.  This  was  pointed  out  some  years  ago  by 
Walkoff  in  an  elaborate  study  of  the  bones  by  the  use  of  the 
.r-rays.  By  this  means  he  showed  that  the  plates  of  cancel- 
lous bone  in  certain  areas  had  a  definite  arrangement  which 
was  related  to  the  attachments  of  certain  muscles.  From 
the  examination  of  sections  of  the  mandible  it  will  be  found 
that  not  only  is  the  general  form  of  the  bone  determined  by 
the  forces  to  which  it  has  been  subjected,  but  also  that  its 
minute  inner  structure  is  definitely  arranged  with  reference 
to  these  forces.  The  direction  and  arrangement  of  the 
plates  of  cancellous  bone  are  continually  changed  and  rebuilt 
to  readjust  them  to  the  support  of  new  conditions  (Fig.  2.32). 

Cribriform  Plates. — The  alveoli  or  sockets  into  which  the 
roots  of  the  teeth  fit  are  bounded  by  a  thin,  definite  wall, 
which  is  pierced  by  a  great  many  openings.  These  have 
been  called  the  cribriform  plates,  or  sieve-Hke  plates.  They 
unite  the  cortical  plates  of  the  bone  at  the  border  of  the 


378     THE   TEETH    AND    DEVELOPMENT  OF   THE  FACE 

alveolar  process,  and  are  fused  with  it,  on  their  labial  and 
lingual  sides.  The  cribriform  plates  forming  the  walls  of  the 
alveoli  are  really  made  up  of  a  thin  layer  of  subperidental 
bone,  which  has  been  built  on  to  the  plates  of  cancellous 
bone,  to  attach  the  fibers  of  the  peridental  membrane  (see 
p.  299).    Within  the  substance  of  the  bone  and  surround- 

Fia.  287 


The  distribution  of  bone  in  the  alveolar  process. 


ing  the  course  of  the  inferior  dental  artery  and  nerve  is 
found  what  Cryer  has  called  the  cribriform  tube.  This 
extends  from  the  point  where  the  arteries  and  vein  enter 
the  substance  of  the  bone  on  the  lingual  surface  of  the 
ramus,  posterior  to  the  alveolar  process  and  below  the 
oblique  line,  and  extends  through  the  cancellous  portion  of 


RELATION  OF   THE   TEETH   TO   THE  BONE        379 

the  body  of  the  bone,  emerging  at  the  mental  foramina. 
It  is  really  a  rather  definite  arrangement  of  the  plates  of 
cancellous  bone  around  the  vessels  and  the  nerves. 

Alveolar  Process. — If  the  adult  alveolar  process  as  seen  in 
the  skull  is  examined,  it  is  apparent  that  the  bone  is  arranged 
so  as  to  give  the  greatest  support  T\dth  the  least  possible  bulk, 
and  where  there  is  an  increase  in  bulk  it  is  to  meet  some 
special  force  (Fig.  287).  The  incisors  and  cuspids  are  used 
chiefly  to  bite  off  pieces  of  food,  and  when  the  food  cannot  be 

Fig.  2S8 


Skull  of  orang-outang. 


bitten  it  is  torn  and  wrenched  away.  This  puts  a  heavy 
strain  in  all  directions  on  the  roots  of  the  teeth,  which  must 
be  suppoited  by  the  bone.  For  this  reason  the  roots  of  the 
incisors  are  usually  well  covered  "^ith  bone  through  their 
entire  length.  The  cuspid  root  is  long  and  the  upper  portion 
of  it  so  well  supported  in  the  bone  at  the  side  of  the  nose 
and  toward  the  orbit  that  the  most  convex  portion  of  it  is 
sometimes  uncovered.  In  animals  that  use  the  incisors 
largely  for  tearing,  wrenching,  and  fighting,  the  bone  is 


380       THE   TEETH   AND  DEVELOPMENT  OF   THE  FACE 

greatly  thickened  over  the  incisor  roots,  as  is  shown  in  the 
skull  of  the  orang  (Fig.  288). 

In  the  upper  molars  the  spreading  of  the  three  roots  gives 
abundant  support  against  the  direct  forces  of  occlusion. 
The  grinding  motions  bring  lateral  pressure  against  the 
inclined  planes  of  the  cusps,  which  is  met  by  a  thickening 
of  the  process  in  its  occlusal  third  (Fig.  287),  forming  a 
heavier  ring  of  bone,  while  the  buccal  roots  are  often  exposed 
in  their  middle  third.  In  the  molars  the  buccal  incline  of  the 
lingual  cusps  of  the  upper  occlude  with  the  lingual  incline 
of  the  buccal  cusps  of  the  lower  when  the  jaws  are  brought 
squarely  together,  and  in  the  giinding  motions  the  outward 
pressure  on  the  lower  molars  is  supported  by  the  great  mass 
of  the  bod}^  of  the  bone,  while  the  inward  pressure  is  sup- 
ported by  a  thickening  of  the  occlusal  third,  as  the  entire 
alveolar  process  projects  lingually  from  the  body  of  the  bone. 
In  the  examination  of  any  collection  of  skulls,  the  amount 
and  arrangement  of  the  bone  of  the  alveolar  process  will  be 
found  to  be  an  indication  of  the  masticatory  habits  of  the 
individual. 

In  examining  the  sections  through  the  bone  of  the  alveolar 
process,  the  adaptation  of  the  arrangement  of  bone  to  the 
force  to  be  sustained  should  be  constantly  kept  in  mind. 

Influence  of  Mechanical  Conditions  in  Evolution. — Professor 
E.  D.  Cope,^  in  a  long  treatise  on  "The  Mechanical  Causes 
of  the  Development  of  the  Hard  Parts  in  Mammals,"  has 
elaborated  the  fact  that  the  bones  of  the  skeletons  of  all 
mammals  have  been  influenced  in  their  development  by 
mechanical  conditions,  and  that  their  present  forms  are 
adaptations  to  physical  environment.  In  this  he  states, 
as  a  general*  principle  of  structure,  that  the  bone  is  most 
dense,  but  least  in  amount,  on  the  side  in  the  direction  toward 
which  forces  have  been  exerted  in  development,  and  less 
dense,  but  greater  in  amount,  on  the  sides  from  which  the 
forces  have  been  exerted.  These  statements  should  be 
applied  in  the  study  of  all  the  sections  shown. 

1  Journal  of  Morphology,  1888. 


RELATIOX  OF   THE   TEETH   TO   THE  BONE        381 

An  old  dry  mandible  was  sawed  through  in  the  positions 
indicated  in\he  illustration  (Figs.  289,  290,  and  291). 

FiQ.  289 


Fig.  290 


Human  mandible,  showing  form  of  the  bone  and  the  positions  from  which 
sections  were  cut. 

The  portion  containing  the  bicuspid  and  molar  on  the 
left  side  was  ground  through  the  molar  to  obtain  a  section 


382      THE  TEETH    AND   DEVELOPMENT  OF   THE  FACE 


Fio    291 


Human  mandible,  showing  form  of  the  bone  and  the  positions  from  which 
sections  v/ere  cut. 

Fig.  292 


Ground  section  through  the  mandible  -where  the  bicuspid  had  been  extracted. 


RELATION  OF  THE  TEETH  TO  THE  BONE       383 

Fig.  293 


n-y. 


'    /  -  f  4; 


Transverse  sections  through  the  roots  of  two  bicuspids  and  the  first  molar, 
showing  distribution  of  bone. 


384      THE  TEETH    AND    DEVELOPMENT  OF   THE  FACE 

l)arallel  with  the  axis  of  the  tooth.  The  portion  between 
the  alveohis  and  second  l)iciispid  on  the  left  side  was  ground 
vertically  through  the  area  where  the  first  bicuspid  had 
been  (Fig.  292).  The  portion  on  the  right  side  containing  the 
two  bicuspids  and  molar  was  ground  to  give  three  sections 
at  right  angles  to  the  roots — one  in  the  gingival  third,  one 
about  the  middle  of  the  root,  and  one  just  at  their  apices 
(Pig.  293).  The  distal  portions  of  the  bone  were  decalcified 
and  sections  cut  through  the  alveoli  of  the  second  and  third 
molars  (Figs.  294  and  295). 

The  Distribution  of  Bone  in  the  Mandible. — In  Chapter 
XVIII,  on  Bone,  it  was  stated  that  the  arrangement  of  the 
layers  in  the  tissue  could  be  read  as  a  record  of  the  manner  of 
formation.  In  the  examination  of  these  sections  the  arrange- 
ment of  the  lamellae  is  to  be  studied  in  this  way,  as  well  as 
the  distribution  of  the  varieties  of  bone.  Where  the  bicus- 
pid had  been  extracted  the  alveolus  has  been  filled  with 
fairly  compact  bone,  rounding  over  the  border  of  the  process. 
The  section  ground  through  this  position  shows  the  buccal 
and  lingual  cortical  plates  in  U  shape.  The  two  plates  are 
braced  together  across  the  central  portion  by  spicules  of 
cancellous  bone.  At  the  occlusal  border  the  outline  of 
the  old  alveolus  can  still  be  seen  by  studying  the  section 
carefully  with  the  microscope.  After  the  extraction  of  the 
tooth  the  socket  was  first  filled  with  connective  tissue,  which 
was  later  transformed  into  bone,  joining  that  of  the  alveolar 
wall.  At  A,  near  the  lower  border,  the  subperiosteal  bone  is 
found  to  be  very  thick,  the  bone  evidently  growing  in  that 
direction.  At  B,  near  the  occlusal  border  on  the  lingual 
side,  there  have  evidently  been  absorptions  of  the  surface, 
removing  the  Haversian  system  bone,  and  then  a  few  layers 
of  subperiosteal  bone  have  been  reformed  on  the  surface. 

Fig.  296  shows  a  ground  section  through  the  molar.  The 
cribriform  plates  lining  the  alveoli  join  the  cortical  plates  at 
the  border  of  the  process.  On  the  lingual  side  the  wall  of  the 
process  is  very  thin,  but  is  thickened  in  the  occlusal  third 
to  support  the  tooth  against  force  exerted  lingually.  On 
the  buccal  side  the  cribriform  plate  of  the  alveolar  wall  is 


RELATION  OF   THE   TEETH   TO   THE  BONE        385 

Fig.  294 


3c%     THE   TEETH   AND    DEVELOPMENT  OF   THE  FACE 


Fig.  295 


RELATION  OF  THE  TEETH   TO   THE  BONE       387 

connected  with  the  cortical  plate  b}^  spicules  of  cancellous 
bone.  Below  the  apex  of  the  root  the  cortical  plates  are  con- 
nected by  cancellous  bone  in  which  the  medullary  spaces  are 

Fig.  296 


A  section  ground  through  the  first  molar. 


much  larger.  The  same  arrangement  of  the  cortical  plate  and 
its  bracing  is  shown  in  Fig.  294,  which  cuts  between  the  alveoli 
of  the  second  and  third  molar.    Fig.  329  and  Plate  XV  should 


388     THE  TEETH    AXD    DEVELOPMENT  OF    THE  FACE 


Fig.  297 


The  buccal  plate  from  Fig.  293. 
Fig.  29S 


•-•*.*A« !.-«..  TT.  *JV   ■*♦■ 


/-  •>..  :  vr-f^T^^  .J ^  ■-  */..:■  <^:.rr<«>t-_s»»n:*>*-^;r«K!-?»j»!«i»«»E«WJ8r«cra^ 


Ift. 


..    ^1- 


-^?.^* 


The  lingual  plate  from  Fig.  293. 


RELATIOX  OF   THE   TEETH   TO   THE  BOXE 


189 


be  studied  in  this  connection,  remembering  that  the  bone 
has  been  formed  and  shaped  by  formation  of  subperiosteal 
bone  on  its  surface  and  subperidental  bone  at  the  border  of 
the  process  and  their  transformation  into  Haversian  system 
and  cancellous  bone. 

Fig.  293  is  cut  transversely.  Notice  that  the  gingival 
section  has  been  turned  over  in  mounting.  Observe  the 
cribriform  plates  forming  the  walls  of  the  alveoli,  and  the 
way  these  are  braced  against  each  other  and  the  cortical 

Fig.  299 


The  bone  between  the  alveoli  cf  the  mesial  and  distal  roots  of  the  6rst  molar, 
from  Fig.  293. 


plates  by  bands  of  cancellous  bone.  In  accordance  with  the 
principles  noted,  the  buccal  plate  is  thin  and  very  compact, 
while  the  hngual  plate  is  much  thicker,  but  more  open  in 
structure,  and  the  direction  of  growth  has  been  toward  the 
buccal  as  the  arch  of  the  jaw  increased  in  size.  Fig.  297 
shows  the  buccal  plate  with  higher  magnifications.  Fig.  298 
the  lingual  plate,  and  Fig.  299  the  bone  separating  the  alveoli 
for  the  mesial  and  distal  roots  of  the  molar.  The  third  figure 
of  this  series  shows  only  the  tip  of  the  distal  root  of  the 


390     THE  TEETH    AND    DEVELOPMENT  OF    THE  FACE 

molar,  but  the  arrangement  of  plates  of  cancellous  bone 
between  the  cortical  plates  is  nicely  shown. 

The  Maxilla. — In  the  maxilla  the  arrangement  is  exactly 
on  the  same  plan,  the  details  being  different  because  of  the 
difference  in  the  shape  of  the  bone. 


THE  GROWTH  OF  THE  JAWS 

It  has  long  been  noted  that  at  birth  the  mandible  is 
straight,  and  with  the  eruption  of  the  teeth  the  ramus 
develops  and  the  body  increases  in  size.  In  this  process  the 
thickness  of  the  bone  is  increased  from  the  mental  foramina 
to  the  alveolar  border,  and  the  body  of  the  bone  approaches 
a  right  angle  with  the  ramus.  When  the  teeth  are  lost  or 
lose  their  function  the  alveolar  process  is  destroyed  and  the 
bone  reduced  in  thickness  from  above  downward  until 
the  mental  foramen  comes  to  lie  on  the  upper  surface  of  the 
bone.  The  mandible  performs  two  functions,  a  respirator}^ 
and  a  masticatory  function,  and  it  should  be  remembered 
that  these  are  influential  in  its  development.  The  object 
of  this  section  is  to  give  some  conception  of  the  direction  of 
growth  in  the  development  of  the  bones  of  the  face  and 
the  way  in  which  the  changes  are  brought  about. 

This  can  best  be  done  by  studying  the  series  of  skulls 
from  childhood  to  old  age,  in  which  the  outer  cortical  plate 
has  been  removed  so  as  to  show  the  developing  teeth  in  their 
crypts  and  the  relation  of  the  forming  teeth  to  those  already 
in  occlusion  (Figs.  300  to  314).  At  birth  all  of  the  teeth 
except  the  second  and  third  molars  have  begun  to  develop, 
and  their  tooth  germs  are  lying  embedded  in  the  cancellous 
substance  of  the  maxilla.  In  the  upper  jaw  they  occupy 
almost  all  of  the  space  to  the  floor  of  the  nose  and  orbit, 
and  there  is  little  if  any  indication  of  the  maxillary  sinus 
(Fig.  300).  Each  tooth  germ  is  enclosed  in  a  separate 
crypt,  the  wall  of  which  is  formed  by  a  cribriform  plate. 
The  walls  of  the  crypts  are  braced  against  each  other  and 
the  cortical  plates  of  the  maxillae  by  spicules  of  cancellous 


THE  GROWTH  OF  THE  JAWS 

Fig.  300 


391 


MaxilliB  at  about  eight  inonlhs  after  birth,  showing  the  unerupted  tooth. 


392      THE   TEETH    AND    DEVELOPMENT  OF    THE  FACE 

hone  surrounding  medullary  spaces.  As  the  tooth  develops 
within  its  crypt,  pressure  is  exerted  and  the  crypt  wall  is 
pushed  backward  through  the  cancellous  bone. 

Growth  Force.^ — The  force  exerted  by  the  growing  tooth  is 
the  result  of  the  multiplication  of  cells  in  the  tooth  germ, 
and  is  exactly  comparable  to  the  forces  exerted  by  multi- 
plication of  cells  in  any  position.  For  instance,  the  force 
exerted  by  the  multiplication  of  the  cells  in  the  rootlet  of  a 

Fig.  302 


Maxillae  at  about  one  year. 


plant  is  sufficient  to  force  pebbles  aside  and  make  an 
opening  through  hard  packed  earth.  Some  attempts  have 
been  made  to  measure  the  amount  of  force,  but  we  can  only 
say  that  it  appears  to  be  considerable,  acting  through  short 
range.  How  this  force  is  generated  has  been  a  matter  of 
much  speculation  and  investigation  It  shows  some  points 
of  similarity  with  the  swelling  of  wood  fibers  when  water 
is  added.    It  apparently  is  related  to  osmosis,  and  has  some 


THE  GROWTH  OF   THE  JAWS 


393 


direct  relations  to  blood  pressure.  It  is  certainly  a  very 
complicated  matter,  with  chemical  affinities  at  the  bottom 
of  it. 

Forces  Influencing  Bone  Growth. — While  the  growing  tooth 
germs  are  producing  force  which  causes  conditions  of  stress 
of  the  cortical  plates,  the  growth  of  the  tissues  within  the 
mouth — the  tongue  and  the  associated  organs — is  exerting 


Fig.  .-303 


Maxillae  at  one  and  one-half  years. 

pressure  upon  the  lingual  surfaces  of  the  bone.  The  muscles 
attached  to  their  surfaces  transmit  force  to  the  bone  through 
the  periosteum,  and  the  functions  of  mastication,  deglu- 
tition, and  respiration  are  acting  upon  them.  All  of  these 
are  mechanical  stimuli,  to  which  the  connective-tissue  cells 
respond.  In  all  the  process  of  development  the  growth  is 
the  result  of  all  the  forces  to  which  the  bones  are  subjected, 
perfectly  distributed  through  the  substance  of  the  bone  bv 


394      THE   TEETH    AND    DEVELOPMENT  OF    THE  FACE 

the  agency  of  normal  occlusion.  Any  lack  of  harmony  in 
the  proportion  of  these  forces  may  allow  the  teeth  to  meet, 
when  they  erupt,  outside  of  the  normal  influence  of  their 
cusps,  causing  the  beginning  of  malocclusion.  Any  mal- 
occlusion disturbs  the  balance  in  the  distribution  of  forces, 

Fia.  304 


Maxillae  in  the  second  year,  showing  the  relation  of   the  erupting  teeth.      Note   the 
relation  of  the  crypt  of  the  second  molar  to  the  inferior  dental  canal. 


and  results  in  a  disturbance  of  the  development  of  bone, 
which  progresses  during  the  entire  period  of  development. 
This  must  result  in  the  lack  of  balance  in  the  proportions 
of  the  features  which  will  be  proportionate  to  the  mal- 
occlusion. 


THE  GROWTH  OF  THE  JAWS 


395 


It  has  been  natural  and  almost  inevitable,  because  of 
their  hardness,  to  think  of  bones  as  sohd  and  unchanging. 
In  the  study  of  these  skulls  the  bones  of  the  face  must  be 
viewed  not  as  solid  and  rigid,  but  as  containing  millions 
of  active  cells  which  are  continually  building  and  rebuilding 
their  substance. 

Fig.  305 


The  complete  temporary  dentition  (about  three  years),  showing  the  relation 
of  the  developing  permanent  teeth. 


Usually  somewhere  between  the  seventh  and  ninth  months 
after  birth  the  growth  of  the  central  incisors  causes  the  absorp- 
tion of  the  roof  of  their  crypts,  and  the  tooth  moves  occlusally, 
cutting  through  the  soft  tissues  (Fig.  301).  The  formation  of 
cementum  on  the  surface  of  the  root  and  of  bone  on  the  wall 
of  the  crypt  attach  the  connective-tissue  fibers  and  form  the 
beginnhig  of  the  peridental  membrane.  As  the  tooth  moves 
occlusally  the  bone  grows  up  around  it  from  the  circumference 


396     THE   TEETH    AND    DEVELOPMENT  OF   THE  FACE 

of  the  crypt  wall,  converting  it  into  the  wall  of  the  alveolus. 
The  root  is  not  fully  formed  and  the  conical  pulp  filling 
the  funnel-like  end  exerts  force  by  the  multiplication  of 
cells  and  the  blood  pressure,  which  cause  the  tooth  to  move 
occlusally  and  the  bone  to  grow  in  that  direction.  At  the 
same  time  the  pressure  of  tongue  and  lips  exerts  pressure  on 

Fig.  306 


The  complete  temporary  denlition  and  the  first  permanent  molar.     Note  the 
relation  of  the  bicuspids  to  the  temporary  molars.    (In  the  seventh  year.) 


the  surfaces  of  the  tooth  and  bone,  influencing  the  direction 
of  bone  growth.  The  jaw  increases  in  thickness  in  the  occlusal 
direction  and  grows  forward  and  outward.  At  the  same 
time  the  growth  of  each  successively  distal  tooth  is  exerting 
pressure  upon  those  already  erupted,  causing  them  to  move 
farther  in  the  occlusal  direction.  In  Figs.  303  and  304  notice 
the  way  in  which  the  crypt  walls  are  pushed  downward  by  the 


THE  GROWTH  OF   THE  JA^yS 


397 


development  of  the  tooth  root  until  the  inferior  dental  nerve 
lies  between  the  floor  of  the  crypt  and  the  cortical  plate  of  the 
lower  border.     In  this  way  enough  pressure  may  be  produced 


Fig.  307 


Front  view  of  the  skull  shown  in  Fig.  303.       Note   the   relation   of   the   permanent 
incisors  and  cuspids  to  each  other  and  the  roots  of  the  temporary  teeth. 


398     THE   TEETH    AND   DEVELOPMENT  OF    THE  FACE 

to  cause  reflex  nervous  symptoms,  which  commonly  precede 
the  eruption  of  the  temporary  molars,  and  so  development 
continues  until  all  of  the  temporary  teeth  are  in  place.  About 

Fig.  308 


Dentition  in  the  eighth  year.     Note  the  position  of  the  cuspids  and  compare 
with  Fig.  310. 


THE  GROWTH  OF   THE  JAWS 


399 


the  sixth  year  the  first  permanent  molars  take  their  place  at 
the  distal  of  the  temporary  teeth  and  their  cusps  interlock 
(Fig.  305).     The  importance  of  these  teeth  can  scarcely  be 


Fig.  309 


The  left  side  of  the  skull,  shown  io  Fig.  30S 


400      THE   TEETH    AND    DEVELOPMENT  OF   THE  FACE 

overstated.  They  are  not  only  to  be  the  chief  means  of 
mastication  during  the  period  in  which  the  temporary  teeth 
are  lost  and  replaced  by  their  successors,  but  they  are  to 

Fig.  310 


Dentition  in  the  eleventh  year.     Note  the  growth  of  the  cuspids  and  bicuspids. 
The  second  molar  is  about  to  erupt. 


THE  GROWTH  OF   THE  JAWS 


401 


maintain  the  relation  of  the  jaws  to  each  other.  The  way 
in  which  these  teeth  lock  determines  the  balance  between  the 
forces  exerted  by  the  action  of  the  muscles  attached  in  the 
region  of  the  ramus,  and  those  in  the  region  of  the  svmphvsis 
(Fig.  306). 

Fig.  311 


Dentition  in  the  thirteenth  year.    Note  the  relation  of  the  bicuspid  crown  to  the 
roots  of  the  lower  temporary  molar. 


A  deviation  from  the  normal  relation  of  these  teeth  will 
entirely  change  the  direction  of  the  forces,  and  will  be  mani- 
fested by  a  modification  in  the  development  in  the  bone. 
In  the  skull  at  this  period  the  bicuspids  are  seen  lying  below 
the  temporary  molars,  and  the  second  molar  developing  at 
26 


402       THE   TEETH  AND  DEVELOPMENT  OF  THE  FACE 

the  distal  of  the  first.  Their  growth  is  transmitted  through 
the  teeth  to  the  alveolar  process,  and  the  addition  of  bone 
results.   The  same  skull  viewed  from  in  front  (Fig.  307)  shows 

Fig.  312 


The  dentition  of  a  young  adult.    The  third  molars  have  not  erupted. 
(About  fifteen  years.) 


THE  GROWTH  OF   THE  JAWS 


403 


the  relation  of  the  permanent  incisors  and  cuspids  to  the  tem- 
porary ones.  In  the  lower  jaw  the  temporary  centrals  have 
been  lost  and  the  permanent  ones  are  forcing  their  way 
between  the  temporary  laterals.    The  crowns  of  the  centrals 


Fig.  313 


Adult  dfentition.     Note  the  distance  from  the  apices  of  the  incisors  to  the  lower 
border  of  the  mandible  and  the  floor  of  the  nose. 


are  wider  than  those  of  the  teeth  that  were  lost,  and  they 
consequently  exert  pressure  upon  the  mesial  surfaces  of  the 
laterals,  pushing  them  apart  and  carrying  them  upward  and 
forward. 

Studv  the   relation  of  the   lower  centrals,   laterals,  and 


404        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

cuspids  in  the  development  of  the  arches  at  from  six  to  ten 
years.  Notice  that  the  roots  of  the  central  are  not  fully 
formed,  that  the  lateral  lies  to  the  lingual  of  the  temporary 
lateral  root,  and  with  its  mesio-occlusal  angle  below  the 
distal  surface  of  the  central.  The  development  of  the  cuspid 
has  pushed  the  crypt  floor  through  the  cancellous  bone 
until  it  has  reached  the  solid  cortical  plate,  and  still  the  for- 
mation of  the  crown  is  not  quite  completed.    The  six  teeth 

Fig.  314 


Edentulous  jaws,  showing  loss  of  alveolar  process. 


form  a  triangle  of  which  the  centrals  are  the  apex,  and  the 
cortical  plates  from  cuspid  to  cuspid  the  base.  The  com- 
pletion of  the  roots  of  these  teeth  will  carry  the  temporary 
teeth,  alveolar  process  and  all,  upward,  forward,  and  out- 
ward, thus  increasing  the  distance  from  the  mental  foramen 
to  the  symphysis  and  enlarging  the  arc  of  the  law  from 
cuspid  to  cuspid. 

In  the  same  skull  notice  the  relation  of  the  upper  incisors 
and  cuspids  to  the  corresponding  temporary  teeth.   They  lie 


THE  GROWTH  OF  THE  JAWS  405 

to  the  lingual  of  the  roots  of  the  temporary  teeth,  the  lateral  a 
little  to  the  lingual  of  the  central  and  cuspid.  The  cuspid  has 
pushed  back  the  floor  of  its  crypt  until  it  is  braced  against 
the  solid  bone  at  the  base  of  the  malar  process.  The  growth 
of  these  teeth  will  first  cause  the  temporary  teeth  to  move 
occlusally,  the  bone  growing  from  the  border  of  the  process 
to  follow  them.  In  this  growth  the  distance  from  cuspid 
to  cuspid  is  increased  and  spaces  appear  between  the  tempo- 
rary incisors  some  time  before  they  are  lost. 

If  such  spaces  do  not  appear,  the  development  is  not 
progressing  normally,  and  artificial  force  should  be  applied 
to  stimulate  bone  growth.  If  this  is  not  done  the  permanent 
teeth  are  sure  to  come  in  more  or  less  rotated  and  out  of 
position. 

In  Figs,  308  and  309  the  incisors  have  been  pushed  ofi' 
and  the  permanent  ones  are  beginning  to  move  occlusally. 
Notice  the  relation  of  the  floor  of  the  crypt  to  the  floor  of 
the  nose,  and  the  root  has  scarcely  begun  to  develop.  In  the 
adult  skull  (Fig.  302)  there  is  almost  as  much  space  from  the 
apex  of  the  root  to  the  floor  of  the  nose  as  there  is  now  from 
the  border  of  the  alveolar  process  to  the  floor  of  the  nose. 
The  result  of  the  growth  of  the  cuspids'  roots  is  shown  by 
comparing  Figs.  307  and  308  with  Fig.  310. 

The  Importance  of  Proximal  Contact. — The  proper  contact 
of  the  teeth  upon  their  proximal  surfaces  is  necessary  for 
this  development.  If,  for  instance,  the  mesial  angle  of  the 
lower  lateral  fails  to  engage  with  the  distal  surface  of  the 
central,  but  slips  by  to  the  lingual,  the  growth  of  the  cuspid 
will  push  it  farther  and  farther  past  the  central  instead  of 
enlarging  the  arch.  One  of  the  cogs  in  the  mechanism  has 
slipped,  and  the  growth  of  bone  cannot  later  be  expected  to 
make  room  for  the  crowded  teeth. 

In  the  next  stage  of  groT\i:h  the  increase  in  size  is  from 
the  mental  foramen  to  the  ramus,  and  is  largely  influenced 
by  the  development  of  the  roots  of  the  bicuspids  and  the 
second  molars.  Figs.  309  and  310  show  the  relation  of 
the  second  molar  to  the  distal  surface  of  the  flrst,  and 
it  will  be  seen  that  its  growth  exerts  force  upon  the  first 


406       THE   TEETH  AND  DEVELOPMENT  OF   THE  FACE 

Fig.  315 


Fig.  316- 


Figs.  315  and  316  were  photogiaphed  in  the  same  relative  size,  to  show  the  amount 
and  direction  of  growth,  with  the  development  of  the  full  permanent  dentition. 


THE  GROWTH  OF  THE  JAWS 

FiQ.  317 


407 


Fig.  318 


Figs.  317  and  31S  -were  pliotographet:  in  tlio  same  relative  size,  to  shew  the  amount 
and  direction  of  growth,  with  the  deveiupmeut  of  the  full  permanent  dentition. 


I'la.  319 


Fig.  320 


Figs.  319  and  320  were  photographed  in  the  same  relative  size,  to  show  the  amount 
and  direction  of  growth,  with  the  development  of  the  full  permanent  dentition. 


Fig.  321 


Fig.  322 


Figs  321  and  322  were  photographed  in  the  same  relative  size,  to  show  the  amount 
and  direction  of  growth,  with  the  development  of  the  full  permanent  dentition. 


410       THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

molar,  and  this  is  transmitted  through  the  arch  by  means 
of  proximal  contact.  Notice  the  inclination  of  the  bicuspid 
roots,  which  help  to  carry  the  growth  in  the  same  direction. 
After  the  second  molar  is  in  place  the  growth  of  the  third 
should  exert  the  same  force  and  room  be  provided  for  it  (Fig. 


Fig  323.— Two  years. 


Fig.  324. — Three  years. 


Fig.  325.— Six  years.  Fig.  326.— Ten  years. 

Maxillae  photographed  from  the  median  line  in  the  same  relative  size,  to  show  the 

amount  and  direction  of  growth. 


THE  GROWTH  OF  THE  JAWS 


411 


Fig.  327.— Twelve  years. 


Fig.  328.— Adult. 


Maxillae  photographed  from  the  median  line  in  the  same  relative  size,  to  show  the 
amount  and  direction  of  growth. 


311).  The  muscular  action  of  the  lips  and  tongue  are  specially 
important  in  these  last  stages  of  growth,  and  particularly  the 
forces  that  are  generated  by  the  action  of  the  muscles  in 
respiration  and  deglutition.  The  activity  of  the  connective- 
tissue  cells  in  the  bone  require  mechanical  stimuli  for  their 
maintenance,  and  as  the  muscular  action  is  vigorous  or 
deficient,  the  growth  of  bone  mil  be  full  and  normal  or 
imperfect  and  unbalanced.  It  appears  often  that  the  bone 
activity  becomes  so  sluggish  that  the  growth  of  the  third 
molar  cannot  produce  the  effect  it  should,  and  it  remains 
impacted.  A  comparison  of  figures  will  show  that  while 
room  has  been  made  for  the  third  molar,  all  of  the  upper 
teeth  have  moved  downward,  forward,  and  outward,  and 
the  lower  ones  upward,  forward,  and  outward.  Compare 
the  distance  from  the  apex  of  the  incisor  roots  to  the  floor 
of  the  nose  and  the  lower  border  of  the  mandible  in  Figs. 
312  and  313. 

This  process  may  be  more  fully  realized  by  comparing 


412       THE   TEETH  AND  DEVELOPMENT  OF  THE  FACE 

the  front  views  of  the  skulls  (Figs.  315  to  322).  They  were 
all  photographed  with  the  same  lens  and  bellows  length, 
so  as  to  make  the  pictures  of  the  same  relative  size  as  the 
skulls.  Notice  the  increase  in  distance  from  the  floor  of  the 
nose  and  the  floor  of  the  orbit  to  the  edges  of  the  upper 

Fig.  329 


Bone  from  the  buccal  plate  of  the  mandible  of  a  young  sheep,  showing  transforma- 
tions of  bone:  1,  subperiosteal  bone;  2,  Haversian  system  bone;  3,  Haversian  system 
bone,  becoming  cancellous. 


incisors,  and  from  the  lower  border  of  the  mandible  to  the 
edge  of  the  lower  incisors.  It  will  be  seen  that  if  the  infant 
mandible  were  placed  in  relation  to  the  adult  mandible  it 
would  lie  entirely  within  the  arch  and  in  the  mouth  cavity, 
while  in  the  upper  the  temporary  incisors  in  Fig.  322  would 
be  some  place  in  the  nasal  cavity.     In  all  of  this  growth  the 


THE  GROWTH  OF  THE  JAWS 


413 


size  of  the  air  spaces  increases  with  the  movements  of  the 
teeth,  the  floor  of  the  nose  and  palate  growing  downward 
and  developing.    This  may  be  shown  in  Figs.  323  to  328,  in 


Fig.  330 


The  record  in  the  arrangement  of  the  lamellae  of  the  growth  of  the  mandibles.      A 
decalcified  section  from  near  the  lower  border  of  a  human  mandible. 


which  the  right  half  of  the  maxilla  has  been  removed  from 
dissected  skulls  and  photographed  from  the  median  line. 

Tissue  Changes  in  the  Physiologic  Movements  of  the  Teeth. 
— All  that  has  been  said  in  regard  to  bone  growth  must  be 


414       THE   TEETH   AND  DEVELOPMENT  OF   THE  FACE 

recalled  in  order  to  obtain  a  conception  of  the  manner  in 
which  these  movements  of  the  teeth  and  the  development 
of  the  bone   are  accomplished.      Bone  laid   down   under 

FiQ.  331 


A  decalcified  section  from  the  lingual  vertical  plate  of  a  human  mandible,  showing 
the  arrangement  of  lamella  as  a  record  of  growth. 


the  periosteum  and  the  peridental  membrane  has  been 
transformed  into  Haversian  system  bone  and  then  made 
cancellous,  as  illustrated  in  Fig.  329,  which  is  taken 
from  the  buccal  plate  of  the  mandible  of  a  young  sheep. 


THE  GROWTH  OF   THE  JAWS 


415 


Reversed  changes  have  also  been  going  on,  the  periosteum 
cutting  into  the  Haversian  bone  by  absorption  and  the 
cancellous  bone  being  condensed  into  Haversian  system 
bone.    These  changes  leave  a  record  in  the  arrangement  of 


Fro.  332 


Cancellous  bone  from  a  decalcified  section  of  a  human  mandible,  showing 
reconstructions  to  change  the  direction  of  the  spicules. 


the  lamellse,  and  may  be  studied  in  decalcified  sections 
(Figs.  330  to  333).  Even  the  direction  of  the  spicules  of 
cancellous  bone  are  being  constantly  changed  by  absorptions 
and  rebuilding  to  adjust  them  to  changes  of  stress. 


416       THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

While  the  temporary  teeth  are  moving  occlusally,  bone  is 
laid  down  under  the  peridental  membrane  at  the  border  of  the 
alveolar  process,  which  is  at  once  cut  out  by  absorptions  and 
replaced  by  Haversian  system  bone  (Fig.  217).  The  alveolar 
process  becomes  a  veritable  patchwork,  as  shown  in  Figs. 
234  and  335.  The  permanent  tooth  developing  in  its  crypt 
produces  conditions  of  pressure,  and  osteoclasts  appear  in 

Fici.  3;33 


Decalcified  section  of  cancellous  bone  from  a  human  mandible,  showing  absorptions 
and  rebuildings,  changing  the  direction  of  the  spicules. 


all  the  medullary  spaces,  around  and  above  the  crypt,  and 
through  the  alveolar  process,  as  well  as  on  the  crypt  wall. 
They  are  more  active  in  the  medullary  spaces,  cutting  away 
the  spicules  of  bone,  thinning  and  cutting  apart  the  crypt 
wall,  and  allowing  it  to  be  bent  and  pushed  back. 

Fig.  336  shows  the  alveolar  process  on  the  lingual  side 
of  the  temporary  incisor,  and   illustrates  the  enlargement 


THE  GROWTH  OF  THE  JAWS 


417 


of  the  medullary  spaces  preparatory  to  the  eruption  of 
the  permanent  tooth.  Fig.  337  shows  the  labial  plate  of  the 
process,  and  notice  that  the  bone  is  being  formed  under  the 


Fig.  334 


A  longitudinal  section  through  the  tip  of  the  alveolar  process  of  a  temporary  tooth 
about  ready  to  be  lost:  D,  dentine;  Cm,  cementum,  showing  absorption  and  rebuild- 
ing; Pd,  peridental  membrane;  B,  bone  growing  occlusally  at  the  border  of  the 
process;  Hb,  rebuilt  Haversian  svstem  bone. 

27 


FiQ   335 


A  longitudinal  section  through  the  temporary  alveolar  process,  which  is  growing 
occlusally  to  follow  the  temporary  tooth.  It  is  from  the  same  series  as  Fig.  334,  but 
shows  more  of  the  bone.  Study  the  absorptions  and  rebuildings,  as  shown  in  the  arrange- 
ment and  character  of  the  lamellae.     Pd,  peridental  membrane:  Po,  periosteum. 


THE  GROWTH  OF  THE  JAWS 


419 


420     THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 


THE  GROWTH  OF  THE  JAWS 


421 


422     THE  TEETH   AND    DEVELOPMENT  OF   THE  FACE 

periosteum  and  at  spots  under  the  dental  membrane,  while 
the  substance  of  the  bone  is  being  destroyed. 

When  the  tooth  is  finally  pushed  off  from  the  gum  all 
but  a  few  bits  of  the  alveolar  process  have  been  destroyed, 
and  as  the  permanent  tooth  comes  through,  bone  formation 
begins  at  the  border,  patching  on  to  the  remains  of  the  old 
process  (Fig.  338). 

In  studying  the  absorption  of  bone  around  the  crypt 
walls,  it  has  been  noted  that  the  osteoclasts  appear  first 
in  the  cancellous  bone  (Figs.  218  and  219),  surrounding  the 
crypts  and  outside  of  it.  Absorptions  here  remove  the  spicules 
which  brace  the  crypt  wall,  and  cut  through  the  wall  in  such 
a  way  as  to  allow  it  to  be  pushed  back  through  the  weakened 
substance.  In  the  same  way  in  the  movements  of  the  teeth, 
absorptions  appear  first  in  the  spaces  outside  of  the  cribri- 
form plates  of  the  alveoli,  until  the  remaining  bone  is  weak- 
ened sufficiently  to  spring  under  the  pressure.  All  of  the 
sections  of  the  mandible  should  be  studied  as  a  record 
of  these  bone  transformations,  and  especially  in  orthodontia 
it  should  be  remembered  that  appliances  are  used  not  to 
push  the  teeth  through  the  bone  as  a  post  would  be  pushed 
through  the  mud,  but  to  supply  mechanical  stimuli  to 
living  cells  whose  activity  will  result  in  bone  growth,  carrying 
the  teeth  into  their  proper  positions,  and  finally  that  teeth 
will  remain  only  in  the  position  in  which  all  of  the  forces 
to  which  it  is  subjected  are  balanced. 


PAET  II 

DIRECTIONS  FOR  LABORATORY  WORK 

(TWENTY-FIVE  PERIODS  IN  THE  LABORATORY) 


INTRODUCTION 

It  is  assumed  in  this  work  that  the  student  has  had  a 
course  in  general  histology,  including  laboratory  work,  that 
he  is  familiar  with  the  technique  of  handhng  the  microscope, 
the  technique  of  staining  and  mounting  sections,  and  that 
he  is  able  to  recognize  at  once  the  elementary  tissues.  The 
same  outfit  is  required  as  for  general  histology,  including 
slides  and  blank  labels  for  them;  cover-glasses;  teasing 
needles;  forceps;  section  lifter;  a  tube  of  balsam;  a  funnel; 
pipette;  filter  paper  and  lens  paper;  6  one-ounce  reagent 
bottles  containing  xylol,  absolute,  95,  and  70  per  cent,  alco- 
hols, hematoxylin,  and  eosin;  at  least  two  chip  butter 
dishes  that  can  be  used  for  staining;  a  box  for  the  slides; 
a  notebook;  a  hard  and  a  soft  drawing  pencil;  a  good  eraser; 
and  a  piece  of  clean,  soft  linen  for  wiping  slides  and  cover- 
glasses. 

Teeth  for  Grinding. — It  is  difficult  to  obtain  satisfactory 
teeth  for  the  grinding  of  microscopic  sections,  and  the 
student  should  bring  to  the  laboratory  a  number  of  suitable 
teeth  from  which  selection  can  be  made.  Old,  dry  teeth  are 
absolutely  useless  for  the  purpose,  however  perfect  their 
structure  may  have  been.  When  a  tooth  has  been  extracted 
for  some  time  the  tissues  dry  out,  giving  up  a  considerable 
amount  of  water,  and  consequently  shrink.  The  shrinkage 
of  dentine  and  enamel  is  unequal,  and  the  result  is  a  cracking 


424  DIRECTIONS  FOR  LABORATORY  WORK 

of  the  tissue.  The  observation  of  the  teeth  in  any  skull  will 
reveal  cracks  in  the  enamel  that  may  be  seen  with  the  naked 
eye,  the  tooth  often  splitting  lengthwise.  Besides  the  cracks 
that  can  be  seen,  the  tissue  is  full  of  microscopic  cracks. 
When  the  grinding  of  sections  from  such  teeth  is  attempted, 
before  the  section  is  reduced  to  sufficient  thinness  for  micro- 
scopic observation,  the  enamel  will  break  to  pieces  and  be 
lost.  A  tooth  that  is  to  be  used  for  grinding  must  be  placed 
in  solution  as  soon  as  it  is  extracted,  and  never  at  any  stage 
of  the  process  be  allowed  to  dry,  until  ready  for  mounting. 
Any  solution  that  will  prevent  decomposition  will  do  for 
this  purpose.  The  best  that  I  have  found  is  a  4  per  cent, 
formaldehyde  in  50  per  cent,  alcohol.  This  may  be  roughly 
prepared  by  diluting  95  per  cent,  alcohol  with  an  equal  volume 
of  water  and  adding  one  part  of  formalin  to  nine  parts  of 
the  diluted  alcohol : 

Alcohol 45  c.c. 

Water 45  c.c. 

Formalin 9  c.c. 

This  solution  not  only  prevents  the  drying,  but  has  a 
hardening  action  on  the  organic  matter,  which  facilitates  the 
grinding.    Teeth  may  be  preserved  in  this  indefinitely. 

Teeth  Required. — From  his  collection  the  student  should 
select  for  grinding  an  incisor  or  cuspid,  a  bicuspid,  and  a 
molar.  The  teeth  should  be  free  from  caries  and  their 
crowns  as  perfect  as  possible. 

The  Relation  of  the  Section  to  the  Crown. — The  practical 
value  of  the  study  of  ground  sections  depends  upon  obtaining 
from  them  a  knowledge  of  enamel  rod  directions  in  relation 
to  the  tooth  crown  as  well  as  the  section.  In  operating  the 
teeth  are  looked  at  from  their  outside  surface,  but  the 
operator  needs  to  see  in  the  enamel  not  simply  a  hard  and 
extremely  dense  tissue,  but  a  tissue  made  up  of  minute  rods 
whose  general  direction  he  knows  beforehand.  If  a  tooth 
is  selected  and  a  section  cut  from  it  in  a  known  position,  and 
the  relation  of  the  section  to  the  crown  remembered,  the 
direction  of  enamel  rods  can  be  placed  in  relation  to  the 


PREPARATION  OF  GROUND  SECTIONS  OF  TEETH     425 

entire  crown  as  well  as  to  the  section.  This  is  one  of  the 
objects  to  be  sought  in  the  making  of  the  outline  drawings. 

Location  of  the  Section. — Having  selected  the  teeth  for 
grinding,  the  next  step  is  to  locate  the  position  and  direction 
of  the  section.  This  must  be  so  placed  as  to  cut  the  enamel 
rods  in  their  length.  The  section  from  the  incisor  or  cuspid 
should  be  ground  labiolingaally,  but  the  section  from  the 
molar  and  bicuspid  may  be  ground  either  buccolingually, 
mesiodistally,  or  diagonally.  The  surface  of  the  tooth 
should  be  considered,  and  the  section  placed  in  an  area  in 
which  the  student  desires  to  discover  the  enamel  rod  direc- 
tions and  the  structure  of  the  tissue.  The  line  of  the  section 
should  now  be  marked  on  the  tooth  with  India  ink  and  a 
fine  pen. 

The  Drawings  of  the  Teeth. — After  marking  the  position  of 
the  section  the  tooth  should  be  carefully  and  accurately 
drawn,  showing  the  position  of  the  section  as  seen  from  the 
axial  and  occlusal  surfaces. 

Grinding  of  the  Section. — Every  institution  should  have  a 
machine  for  the  preparation  of  ground  sections,  but  such  a 
machine  is  too  delicate  an  instrument  to  be  handled  by 
students.  In  the  appendix  will  be  found  a  chapter  written 
by  Dr.  Black  describing  the  grinding  machine  and  the 
technique  of  its  use.  If  one  is  available,  the  student  may  have 
his  sections  ground  for  him  and  returned  ready  to  mount,  or 
he  may  grind  them  himself,  using  the  following  technique: 

Preparation  of  Ground  Sections  of  the  Teeth.— For  this  work 
the  student  should  have  two  large  corundum  stones  not  less 
than  four  inches  in  diameter,  one  of  "C"  and  one  of  "E" 
grit.  Corundum  is  very  much  better  than  carborundum  for 
this  purpose.  In  grinding  the  stone  should  be  kept  revolving 
slowly  and  moistened  with  a  stream  of  water.  Holding  the 
tooth  against  the  flat  side  of  the  coarse  stone  with  the 
fingers,  the  tissues  should  be  rapidly  ground  away  until 
the  position  marked  for  the  section  is  reached,  when  the  fine 
stone  should  be  substituted  and  the  grinding  continued  just 
enough  to  remove  the  scratches.  The  surface  should  now 
be  polished  on  the  Arkansas  stone  until  a  very  perfect  surface 


426  DIRECTIONS  FOR  LABORATORY  WORK 

has  been  obtained.  Wash  the  specimen  clean  and  immerse 
in  several  changes  of  95  per  cent,  alcohol,  and  leave  in  absolute 
alcohol  in  a  closed  bottle  for  several  hours  or  over  night. 
Harden  a  drop  of  balsam  on  the  centre  of  a  clean  slide  by 
warming  it  over  a  Bunsen  burner  to  evaporate  the  xylol. 
When  the  slide  is  cool  the  balsam  should  be  neither  sticky 
nor  brittle.  Now  remove  the  tooth  from  the  alcohol,  wipe 
it  dry,  and,  placing  it  on  the  balsam  with  the  polished  surface 
next  to  the  glass,  gently  warm  the  slide  until  the  balsam  is 
thoroughly  softened,  and  press  the  tooth  down  against  the 
glass  and  clamp  it  firmly  in  position,  using  a  spring  clip. 
Set  it  away  to  harden  thoroughly,  when  the  grinding  may  be 
continued. 

Holding  the  slide  parallel  with  the  surface  of  the  coarse 
stone,  the  tissues  may  be  rapidly  removed  until  the  section 
is  about  as  thin  as  a  calling  card,  when  the  fine  stone  should 
be  substituted  and  the  section  reduced  to  the  required  thin- 
ness. It  should  not  be  more  than  twenty  microns  in  thick- 
ness. In  the  final  stages  progress  of  the  grinding  may  be 
followed  with  a  hand  magnifying  glass.  Finally  the  surface 
should  be  polished  on  an  Arkansas  stone.  The  specimen 
should  now  be  washed  with  alcohol,  the  balsam  removed 
with  xylol,  and  brought  to  the  laboratory  in  95  per  cent, 
alcohol,  where  it  is  to  be  etched  and  mounted  according  to 
the  directions. 

Every  step  in  the  above  technique  is  important  and  must 
be  followed  with  minute  care  and  accuracy.  Not  least 
important  is  the  cleaning  of  the  slide.  It  sometimes  happens 
that  the  section  will  be  loosened  from  the  glass  before  the 
grinding  is  completed.  This  is  usually  due  to  some  fault 
in  the  technique.  When  it  happens  it  is  best  to  finish  the 
grinding  without  attempting  to  refasten  the  section  to  the 
slide.  To  do  this  the  section  should  be  held  against  the  flat 
side  of  the  stone,  using  a  fine-grained  cork,  a  piece  of  box- 
wood, or  some  similar  material.  The  danger  of  breaking 
the  section,  however,  is  much  greater. 

The  Preparation  of  Transverse  Sections  of  the  Root. — For 
this  purpose  one  of  the  flattened  roots  furnishes  the  best 


DRAWINGS  427 

material,  as,  for  instance,  the  mesial  root  of  a  lower  molar, 
the  root  of  a  lower  bicuspid,  or  of  an  upper  second  bicuspid. 
Holding  the  root  in  a  vice  by  the  remains  of  the  crown, 
with  a  metal  saw,  saw  off  the  tip  of  the  root,  removing  an 
eighth  of  an  inch  or  less.  Then  saw  off  as  thin  a  slice  as 
possible.  In  the  same  way  saw^  out  at  least  two  other 
sections,  one  from  the  gingival  and  one  from  the  middle 
third  of  the  root.  These  should  be  dropped  into  a  bottle 
of  formalin-alcohol  until  the  grinding  is  completed.  The 
grinding  is  easily  accomplished  on  the  flat  side  of  the  corun- 
dum stone,  holding  the  section  on  the  finger  or  under  a 
cork.  The  last  grinding  should  be  done  on  the  fine  Arkansas 
stone. 

Transverse  sections  of  the  root  are  easily  ground  and  can 
be  made  very  thin. 

Manner  of  Working  in  the  Laboratory. — In  no  place  in  the 
world  can  time  be  wasted  more  easily  than  in  the  histological 
laboratory.  The  student  should  take  the  attitude  of  an 
original  investigator  and  study  out  the  material  for  himself 
as  far  as  possible,  remembering  that  he  has  a  far  better 
opportunity  than  the  man  who  worked  out  the  details  of 
these  structures.  He  must  constantly  try  to  picture  the 
structure,  and  imagine  how  it  would  appear  if  sectioned  in 
another  direction. 

Drawings. — Drawings  from  the  microscope  are  made  not 
simply  to  occupy  the  student's  time,  nor  as  a  record  of  what 
he  has  done,  but  to  make  observation  more  accurate  and 
detailed,  and  to  fix  the  impressions  of  structure  more  per- 
fectly in  mind.  Many  students  excuse  themselves  for 
careless  and  slovenly  work  by  saying  that  they  are  not 
artists.  Anyone  without  any  knowledge  of  the  principles 
of  art  can  in  a  very  short  time  acquire  the  ability  to  make 
excellent  microscopic  drawings.  A  few  principles  of  pro- 
cedure will  help  greatly.  The  first  of  all  is  that  a  light 
line  can  always  be  made  darker,  therefore  the  drawing  should 
always  be  kept  light  until  the  later  stages. 

After  selecting  a  field,  draw  lightly  the  outline  of  the 
principal  masses  and  then  the  outKnes  of  the  smaller  ones. 


428  DIRECTIONS  FOR  LABORATORY  WORK 

In  this  way  the  proportion  of  objects  in  the  field  and  their 
relation  to  each  other  can  be  maintained.  Never  draw  any 
detail  such  as  individual  cells,  nuclei,  etc.,  until  all  of  the 
outlines  are  completed.  Then  work  in  the  details  in  the 
darker  colored  areas.  The  making  of  the  outlines  is  by  far 
the  most  important  stage  in  the  drawings. 

Each  outfit  should  contain  a  6  H  and  an  H-B  pencil  and  a 
good  eraser,  which  must  be  kept  clean.  The  pencils  should 
be  kept  sharp  and  always  used  with  a  light  touch  upon  the 
paper.  The  beginner  always  tends  to  start  his  drawing  by 
making  a  circle.  This  should  be  avoided,  for  it  is  objects 
that  are  being  studied,  not  fields,  and  in  many  cases  the 
object  cannot  be  bounded  by  a  circle.  There  is  also  a  ten- 
dency to  represent  the  object  smaller  on  the  paper  than  it 
appears  in  the  field. 

The  prime  qualities  in  a  microscopic  drawing  are  accuracy 
and  correctness  of  detail.  The  drawings  are  made  to  show 
all  the  detail  of  structure  that  can  be  observed.  It  often 
happens  that  a  drawing  that  looks  very  well  shows  very 
little  knowledge  of  the  structure  of  the  tissue  which  it 
represents. 

Stencilled  Laboratory  Notes. — In  fifteen  years  of  teaching 
the  author  has  found  stencilled  notes  on  the  daily  work  in 
the  laboratory  of  very  great  assistance.  There  are  always 
variations  in  the  appearance  of  the  material  which  cannot 
be  anticipated  before  the  sections  are  cut.  Very  often  some- 
thing will  be  seen  unusually  well  that  would  not  be  men- 
tioned in  the  text-book.  Difterent  stains  may  have  been 
used  which  would  change  the  appearance  of  the  tissues,  and 
for  all  of  these  things  and  many  others  daily  notes  are  very 
convenient. 


USE  OF  DIRECTIONS  FOR  LABORATORY  WORK 

At  the  beginning  of  the  laboratory  period  the  first  thing 
to  be  done  is  to  read  through  the  directions  for  the  day's  work. 
The  amount  of  work  for  the  dav  is  then  clearlv  in  mind,  and 


ETCHING  AND  MOUNTING  OF  GROUND  SECTIONS     429 

all  the  steps  in  any  procedure  that  is  to  be  undertaken  are 
understood  at  the  beginning.  It  is  necessary  to  divide  the 
time  available,  so  as  to  accomplish  the  work  indicated  for  the 
day. 

PERIOD  I 

Drawings  of  Tooth  Surfaces  Showing  the  Position  of  Sections. 
— The  object  of  these  drawings  is  to  show  the  relation  of  the 
section  to  the  crown  from  which  it  is  ground,  so  that  in 
studying  the  enamel  rod  directions  as  seen  in  the  sections, 
they  may  be  referred  to  the  entire  crown.  The  drawings 
should  be  made  from  five  to  ten  times  natural  size,  and  must 
be  made  accurately  to  scale  (Fig.  339).  Measure  the  length 
and  the  breadth  of  the  tooth  and  lay  out  a  rectangle,  say 
eight  times  these  dimensions,  to  serve  as  a  guide  in  drawing. 
If  the  tooth  is  marked  for  a  buccolingual  section,  stick  the 
apex  of  the  root  on  a  bit  of  wax  and  place  the  tooth  on  the 
table  with  the  buccal  surface  toward  you.  Do  not  change  its 
IDOsition  until  the  drawing  is  completed,  for  to  do  so  would 
change  lights  and  shadows.  After  getting  the  outline  accur- 
ately, work  in  the  shadows  so  as  to  give  the  drawing  round- 
ness. Remember  in  doing  this  that  you  can  always  make  it 
darker,  but  you  cannot  erase  without  injuring  the  neatness 
of  the  drawing.  When  the  drawings  are  completed  the  sec- 
tion is  ready  for  grinding,  which  must  be  done  outside  of 
the  laboratorv,  following  the  directions  in  Introduction  to 
Part  II. 

PERIOD  n 

Etching  and  Mounting  of  Ground  Sections. ^ — At  the  desk  will 
be  found  1  per  cent,  hydrochloric  acid,  dilute  ammonia,  and 
vaseline,  which  are  the  only  reagents  not  included  in  the 
outfit  and  required  for  this  work.  The  sections  are  brought 
to  the  laboratory  ground  and  ready  to  mount.  Fill  one  of 
the  dishes  with  water  and  carefully  wash  the  specimen  free 
from  all  debris  of  grinding.    Dry  the  section  between  filter 


430  DIRECTIONS  FOR  LABORATORY  WORK 


Fig.  339 


D/ST/IL  SURFACE 


BUCCAL 


OCCLUSAL  SURFACE 
BUCCAL    MARGIN 


DISTAL  M. 


MESIAL  M. 


LING 


LINGUAL  M. 


8  DIAMETERS 


Drawings  of  occlusal  and  axial  surfaces  of  a  tooth,  to  show  the  relation  of  the  section 
to  the  tooth.     (Drawn  by  W  .A.  Offil,  1910.) 


ETCHING  AND  MOUNTING  OF  GROUND  SECTIONS     431 

papers,  so  as  to  remove  all  moisture  from  the  surface.  Fill 
one  dish  with  1  per  cent,  hydrochloric  acid  and  the  other 
wdth  dilute  ammonia.  Put  a  very  little  vaseHne  upon  the 
tip  of  the  finger,  and  holding  the  section  by  the  root  portion, 
cover  one  surface  of  the  crown  portion  with  a  very  thin  layer 
of  it.  In  doing  this  the  vaseline  should  be  wiped  from  the 
centre  toward  the  edges  of  the  section,  so  as  to  prevent  it 
from  running  over  on  to  the  other  surface.  The  vaseline  is 
to  confine  the  action  of  the  acid  to  one  surface  of  the  enamel. 
Holding  the  section  by  the  root  portion,  immerse  the  crown 
in  the  dilute  acid  for  thirty  seconds,  or  until  minute  bubbles 
can  be  seen  forming  upon  the  surface.  Remove  and  immerse 
at  once  in  the  dilute  ammonia  for  a  minute.  Remove  the 
vaseline  by  carefully  wiping  the  section  vriih  absolute  alco- 
hol or  ether,  and  immerse  in  95  per  cent,  alcohol.  In  this 
it  should  remain  while  the  slide  and  cover-glass  are  being 
prepared.  Obtain  from  the  desk  a  cover-glass  long  enough 
to  cover  the  entire  section  and  carefully  clean  both  slide 
and  cover-glass.  On  the  centre  of  the  slide  place  a  drop  of 
balsam  that  is  as  long  as  the  section.  Holding  the  slide  over 
a  Bunsen  burner  or  alcohol  flame,  warm  it  gently  so  as  to 
evaporate  the  xylol.  In  this  process  the  drop  will  spread 
out  over  the  slide  and  the  direction  of  spreading  may  be 
guided  by  the  heat.  Allow  the  slide  to  cool  and  test  the 
hardness  of  the  balsam  with  a  teasing  needle  or  the  finger 
nail.  When  cold  the  balsam  should  be  just  soft  enough 
to  take  the  imprint  of  the  needle  or  nail,  but  not  be  sticky. 
If  it  is  sticky  it  must  be  reheated;  if,  on  the  other  hand,  it  is 
brittle  enough  to  chip,  it  must  be  scraped  oft'  from  the 
sUde  and  the  process  tried  again.  In  the  same  way  prepare 
a  film  of  balsam  on  the  cover-glass.  Remove  the  section 
from  the  95  per  cent,  alcohol  and  dry  it  for  a  few  minutes 
in  the  air  (after  wiping  with  filter  paper).  Place  the  section, 
etched  side  up,  upon  the  balsam  on  the  slide,  and  place  the 
cover-glass  on  it  balsam  side  doicn.  Warm  the  slide  gently 
over  the  flame,  while  pressing  the  cover-glass  down  with  the 
handle  of  a  teasing  needle.  As  the  balsam  is  warmed,  the 
slide  and  cover-glass  are  brought  together,  forcing  the  balsam 


432  DIRECTIONS  FOR  LABORATORY  WORK 

out  to  the  edge  of  the  cover-glass  in  all  directions.  All 
excessive  balsam  should  be  squeezed  out  at  the  edges. 
Place  on  the  cover-glass  a  small  piece  of  blotting  paper  or  a 
layer  of  cork,  adjust  some  sort  of  a  spring  clip  and  put  the 
section  away  until  the  balsam  is  entirely  hard.  When  the 
balsam  is  entirely  hard  the  excess  may  be  removed  by 
gently  scraping  with  a  knife  blade  and  wiping  with  xylol. 
The  section  should  now  be  labelled  with  the  name  of  the  tooth, 
the  direction  and  position  of  the  section,  the  student's 
name  and  number,  and  the  date. 

The  mounting  in  hard  balsam  greatly  improves  the  value 
of  the  section,  for  the  dentinal  tubules  and  the  lacunae  of 
the  cementum  are  left  filled  with  air  and  can  be  more 
easily  studied.  Sections  may,  however,  be  mounted  in  the 
ordinary  way,  in  soft  balsam.  If  the  section  is  broken  or 
extremely  thin,  soft  balsam  should  be  used. 

PERIOD  m 

Outline  Drawings  from  Ground  Sections. — The  object  of 
the  outline  drawing  is  the  study  of  the  dental  tissues,  their 
distribution,  portion  of  the  tooth  formed  by  each,  their 
relation  to  each  other,  and  the  coarser  points  of  their  struc- 
ture. To  get  the  value  from  this  work  the  drawings  must  be 
made  very  accurately  to  scale  and  as  large  as  the  note  book 
page  will  allow.  With  the  bole  gauge  or  a  millimeter  rule 
measure  accurately  the  length  of  the  section,  multiply  this 
by  eight  or  ten,  and  mark  the  length  on  a  page  of  the  draw- 
ing book.  Measure  the  width  of  the  section  at  the  point 
of  the  greatest  diameter  and  multiply  this  by  the  same 
factor.  Using  this  for  the  width  and  the  previous  measure- 
ment for  the  length,  lightly  draw  a  rectangle,  which  is  to  be 
used  as  a  guide  in  the  construction  of  the  dra^\^ng.  The 
success  now  of  the  drawing  depends  on  the  accuracy  and 
number  of  the  measurements. 

First  measure  the  vertical  distance  from  the  incisal  edge 
to  the  gingival  line  on  one  side  of  the  section,  and  then  on 
the  other,  and  mark  these  on  the  sides  of  the  rectangle. 


OUTLINE  DRAWINGS  FROM  GROUND  SECTIONS     433 

This  will  give  the  relative  length  of  root  and  crown  and  the 
difference,  if  any,  in  the  position  of  the  gingival  line  on  the 
two  sides.  Measure  the  vertical  distance  from  the  most 
prominent  point  on  the  axial  surface  to  the  incisal  edge  or 
the  tips  of  the  cusps.  And  so  on,  making  every  measurement 
that  can  help  in  the  formation  of  the  drawing.  In  this  way 
the  outline  of  the  section  should  first  be  traced  inside  the 
rectangle,  then  the  dento-enamel  junction,  then  the  pulp 
chamber  is  shown,  and  finally  the  cementum.  Before 
drawing  the  outline  of  the  cementum,  the  section  should  be 
placed  under  the  microscope,  using  the  low  power,  and  the 
cementum  should  be  observed  studying  it  from  the  gingival 
line  on  one  side  of  the  section  to  the  gingival  line  on  the  other. 

It  would  be  a  waste  of  time  to  attempt  to  fill  in  the 
structure  of  the  tissue  of  the  entire  outline,  and  ofily  certain 
things  are  to  be  shown  in  these  drawings.  For  that  reason 
fill  in  three  portions  of  enamel  and  dentine  and  three  portions 
of  cementum  and  dentine,  using  the  low  power  objective. 
Study  first  the  bands  of  Retzius  (page  60),  and  lightly 
indicate  their  direction.  Study  the  enamel  rod  direction, 
beginning  at  the  gingival  line  at  one  side  and  following  it 
around  the  crown  to  the  other  side.  In  a  portion  at  the 
incisal  edge,  or  on  the  occlusal  surface,  indicate  the  rod 
directions,  and  in  the  same  way  show  them  in  a  portion 
near  the  centre  of  the  axial  surface  on  one  side  and  near 
the  gingival  line.  Follow  the  dentinal  tubules  which  end 
next  to  the  portions  of  enamel  which  have  been  filled  in  to  the 
point  where  they  open  into  the  pulp  chamber,  and  indicate 
their  direction  (page  171).  In  the  same  way  fill  in  three 
portions  of  the  cementum  and  the  dentine  under  them — 
one  in  the  gingival  line,  one  near  the  middle  of  the  root,  and 
one  in  the  region  of  the  apex  (Fig.  340). 

If  any  portion  of  the  section  has  been  lost  in  grinding, 
that  portion  should  be  indicated  by  dotted  lines,  and  in  the 
same  way,  if  a  portion  of  the  crown  has  been  lost  by  wear, 
the  original  form  may  be  added  in  dotted  fines. 

OutUne  drawings  should  be  made  from  each  of  the  three 
classes  of  teeth — one  from  the  incisor  or  cuspid,  one  from  a 
28 


434 


DIRECTIONS  FOR  LABORATORY  WORK 


bicuspid,  and  one  from  a  molar,  and  a  laboratory  period 
should  be  devoted  to  each  drawing. 


Fig.  340 


ENAMEL 
---ENAMEL  RODS 

'M~  DENTINE  {TUBULES) 


-PULP  CHAMBER 


CEMENTUM 


Outline  drawing  of  longitudinal  section,  made  as  a  study  of  the  dental  tissues. 
(Drawn  by  E.  J.  Schnaidt.) 


MINUTE  STUDY  OF  ENAMEL  AND  DENTINE     435 


PERIOD  IV 

Isolated  Enamel  Rods. — Obtain  from  the  desk  a  fragment 
of  enamel  which  has  been  broken  in  the  direction  of  the  rods. 
Place  a  drop  of  distilled  water  or  glycerin  on  the  centre  of  a 
clean  slide.  jNIoisten  the  broken  surface  with  a  drop  of 
water  and  lightly  scrape  it  with  the  blade  of  a  broad,  sharp, 
chisel,  holding  the  edge  parallel  with  the  surface  and  the 
shaft  at  right  angles  to  it.  Dip  the  edge  of  the  chisel  in  the 
drop  of  liquid  on  the  slide,  and  the  scrapings  will  be  left. 
Cover  with  a  cover-glass  and  study  with  the  high  power,  using 
a  small  diaphragm.  Fragments  of  enamel  will  be  found 
made  up  of  broken  rods,  some  single  and  others  in  groups. 
Note  the  diameter  of  the  rods  and  the  appearance  of  the 
cross-markings,  which  will  be  seen  if  the  light  is  properly 
adjusted.    Draw  as  seen  with  the  high  power. 

Repeat  this  operation,  using  enamel  that  has  been  immersed 
in  1  per  cent,  hydrochloric  acid  foi  a  number  of  hours.  Com- 
pare the  appearance  of  the  rods  with  those  of  the  former 
specimen  and  make  a  drawing  as  seen  with  the  high  power. 

Find  an  old  tooth  with  a  large  carious  cavity,  remove  the 
softened  dentine  without  touching  the  enamel  if  possible. 
Lightly  scrape  the  whitened  inner  surface  of  the  enamel  next 
to  the  cavity  and  mount  the  scrapings  as  before.  Compare 
the  appearance  of  these  rods  isolated  by  the  action  of  caries 
with  those  of  the  previous  specimen.  Notice  that  the  cross- 
markings  are  more  distinct  and  the  expansions  and  con- 
stiictions  of  the  rods  more  prominent.  Draw  a  few  of  the 
rods  as  seen  ^\'ith  the  high  power,  using  the  small  diaphragm . 


PERIOD  V 

Minute  Study  of  the  Enamel  and  Dentine. — Select  a  field 
from  one  of  the  ground  sections  where  the  specimen  is  very 
thin,  and,  if  possible,  where  the  entire  thickness  of  the 
enamel  plate  can  be  seen  in  one  field  with  the  f  objective. 


436 


DIRECTIONS  FOR  LABORATORY  WORK 


To  select  this  field  all  of  the  enamel  in  the  three  sections 
should  be  carefully  studied  with  the  low  power,  and  the  one 


Fia.  341 


DENTO-ENAMEL 


..^ni\?\ 


Sr«P>GHT     ROI 


'•;t' 


pEI'lNPu      TL'SULI 


High-power  drawing  of  the  enamel.     (Drawn  by  A.  B.  Hopper.  1902-03.) 


MINUTE  STUDY  OF  CEMENTUM  AND  DENTINE     437 

chosen  in  which  the  rods  can  be  seen  best  and  can  be  most 
easily  drawn.  Having  selected  the  field,  study  the  enamel 
with  the  high  power,  beginning  at  the  dento-enamel  junction. 
Note  the  form  of  the  dento-enamel  junction  and  the  relation 
of  the  two  tissues  at  this  point.  Note  the  diameter  of  the 
enamel  rods  and  estimate  it,  using  a  red  blood  corpuscle  as  a 
standard  of  measurement.  Note  the  striation  of  the  enamel 
(page  57).  Using  both  the  low  and  the  high  power,  draw 
as  accurately  as  possible  the  enamel  from  the  surface  to  the 
dento-enamel  junction,  showing  all  the  details  of  structure 
that  can  be  made  out. 

The  drawing  should  be  made  as  long  as  the  page  will 
allow,  and  need  not  be  more  than  an  inch  wide,  and  should 
include  just  enough  of  the  dentine  to  show  the  dento-enamel 
junction  and  the  character  of  the  dentine  at  that  point  (Fig. 
341).  Notice  the  diameter  of  the  dentinal  tubules,  comparing 
them  with  the  red  blood  corpuscles  and  the  enamel  rods. 
Note  the  amount  of  matrix  that  separates  the  tubules. 
Observe  the  forking  and  the  anastomosis  of  the  tubules  as 
they  approach  the  enamel,  and  follow  them  as  far  as  possible. 


PERIOD  VI 

Minute  Study  of  the  Cementum  and  Dentine. — With  the  low 
power  study  the  cementum  in  the  three  specimens,  looking 
for  all  the  details  of  structure  that  can  be  made  out  (see  page 
ISl).  In  the  gingival  portions  and  often  well  toward  the 
apex,  especially  if  the  tooth  is  from  a  young  person,  the 
cementum  will  be  very  thin  and  almost  structureless  in 
appearance.  ^Yith  the  high  power,  fine  lines  parallel  with 
the  surface  may  be  seen,  which  indicate  the  lamellae.  In 
the  apical  portion  the  cementum  becomes  much  thicker,  and 
it  will  be  seen  that  each  layer  is  thicker  and  consequently 
more  easily  seen.  Little  black  spots  looking  like  spiders 
will  be  found  in  larger  or  smaller  numbers.  These  are  the 
lacunae  \\dth  the  canaliculi  radiating  from  them.  They  were 
filled  in  life  by  cement  corpuscles.     Look  for  embedded 


438  DIRECTIONS  FOR  LABORATORY  WORK 


fibers  of  the  peridental  membrane.  In  all  of  this  work  each 
field  should  be  studied  with  both  the  low  and  the  high 
power. 


<*M- 


A.e  Hc\s 


yr 


Fig.  342 


—    CEMENTUM 


4 


\  ^^<^y^^  't^  -   GRANULAR  LAYER 


OF   TOMES 


DENTINE 


PULP  CHAMBER 

Cementum  and  dentine.     (Drawn  by  H.  J.  Lund  and  A.  E.  Hopper.) 


DRAWINGS  OF  TYPICAL  CAVITY  WALLS         439 

The  inner  layer  of  the  cementum  next  to  the  dentine  is 
clear  and  structureless,  and  the  dentine  adjoining  it  appears 
with  the  low  power  as  a  granular  layer  known  as  "the  granu- 
lar layer  of  Tomes."  Studied  with  the  high  power,  the 
appearance  will  be  seen  to  be  caused  by  irregular  spaces  in 
the  dentine  matrix  communicating  with  the  dentinal  tubules 
and  filled  in  life  with  protoplasm  of  the  fibrils.  Compare  the 
dentine  in  the  root  with  that  in  the  crown  (page  171). 

After  studying  all  the  cementum  in  the  three  sections, 
select  three  fields,  one  from  the  gingival,  one  from  the 
middle,  and  one  from  the  apical  portion  of  the  root,  and 
draw  the  tissues  from  the  surface  of  the  root  to  the  pulp 
chamber.  Show  all  the  details  of  structure  that  can  be 
made  out  with  both  low  and  high  powers  (Fig.  342).  With 
the  high  power  search  the  cementum  for  the  record  of 
absorptions  which  have  been  refilled  by  cementum. 


PERIOD  VII 

Drawings  of  Typical  Cavity  Walls. — From  the  molar  or 
bicuspid  section  select  a  field  in  the  region  of  a  groove  or 
pit.  Imagine  a  cavity  to  be  prepared  in  this  position.  To 
help  in  this,  an  ink  line  may  be  made  on  the  cover-glass  by 
using  a  fine  pen  and  Indian  ink,  or  ordinary  ink  to  which  a 
little  sugar  has  been  added.  Xow,  using  both  the  high  and 
the  low  power,  study  the  direction  of  the  enamel  rods  as 
they  appear  in  the  line  of  the  cavity  wall,  and  make  a  drawing 
showing  the  structural  requirements  for  a  good  wall  in  this 
position.  From  any  one  of  the  three  sections  select  a  field 
in  the  gingival  third  of  the  labial  or  buccal  surface  and 
indicate  the  line  of  a  cavity  wall  in  the  same  way.  Study 
with  the  low  and  the  high  powers  the  direction  of  the  enamel 
rods  as  they  appear  in  the  line  of  the  walls  of  the  cavity,  and 
make  a  drawing  showing  the  structural  requirements  for 
good  walls  in  these  positions  (page  80). 


440  DIRECTIONS  FOR  LABORATORY  WORK 


PERIOD  vm 

Outline  Drawings  from  Transverse  Sections  of  the  Root. — 

The  ground  sections  of  the  root  have  been  prepared  and  should 
be  brought  to  the  laboratory  in  solution,  ready  for  mounting. 
The  three  sections  should  be  mounted  together  under  one 
cover-glass,  using  balsam  about  the  consistence  of  molasses. 
The  sections  may  be  studied  at  once,  but  after  the  day's 
work  upon  them  they  should  have  a  spring  clip  adjusted 
to  the  cover-glass  and  be  put  away  until  the  balsam  is  thor- 
oughly hard,  otherwise  they  may  work  out  to  the  edge  of 
the  cover-glass.  With  the  millimeter  gauge  measure  the 
length  and  breadth  of  each  section,  multiply  the  measure- 
ments by  twenty,  and  lay  off  a  rectangle  as  in  making  the 
longitudinal  drawings.  Draw  the  outline  of  the  section  and 
the  pulp  chamber  as  accurately  as  possible  before  studying 
the  section  with  the  inicroscope.  With  the  low  power 
follow  the  dentocemental  junction  around  each  section  and 
draw  it  into  the  outline.  Fill  in  half  of  each  section,  showing 
the  direction  of  the  dentinal  tubules,  the  position  and  char- 
acter of  the  granular  layer  of  Tomes,  the  number  and  posi- 
tions of  the  lacunae,  and  the  other  structural  characteristics 
of  the  cementum.  In  this  study  the  record  of  the  reduction 
of  size  of  the  pulp  chamber  which  may  be  noted  by  changes 
in  the  direction  and  the  character  of  the  dentinal  tubules 
(page  185).  Label  the  section  with  the  name  of  the  root 
from  which  it  was  ground,  your  name,  and  the  date. 


PERIOD  IX 

Study  of  Secondary  Dentine  and  Cementum. — With  the 
low  power  find  a  field  where  there  is  a  distinct  demarcation 
between  dentine  of  earlier  and  later  formation,  and  draw  it 
accurately  with  the  high  power.  Compare  the  size  of  the 
tubules,  their  number,  their  direction,  and  their  diameter  in 
the  earlier  with  the  later  formed  dentine;  is  there  any  con- 


GROUND  SECTIONS  OF  BONE  441 

nection  between  the  tubules  of  the  two  portions?  Find  a 
similar  field  from  a  longitudinal  section  and  study  in  the 
same  way,  making  an  accurate  drawing. 

Search  all  of  the  ground  sections  with  the  low  power  until 
a  field  is  found  where  the  dentinal  tubules  are  cut  trans- 
versely. Adjust  the  high  power  objective  and  study  the 
field.  Notice  that  by  focussing  up  and  down  with  the  fine 
adjustment  the  tubules  seem  to  move  in  a  circle,  showing 
the  spiral  course  through  the  matrix.  Using  a  red  blood 
corpuscle  as  a  standard,  note  the  size  of  the  tubules,  their 
distribution  in  the  matrix,  and  the  amount  of  matrix  sepa- 
rating them.  Look  for  the  appearance  of  Newman's  sheath, 
which  is  that  portion  of  the  matrix  forming  the  immediate 
wall  of  the  tubule.  Draw  accurately  one  field  as  seen  with 
the  high  powder.  Study  the  cementum  from  all  the  ground 
sections  for  an  area  showing  absorption  and  rebuilding,  and 
if  found,  draw  one  field  with  the  high  power.  Draw  five  or 
six  lacunae  with  their  canahculi  as  seen  with  the  high  power, 
selecting  as  great  a  variety  of  forms  as  possible. 


PERIOD  X 

Ground  Sections  of  Bone.— From  a  shaft  of  a  femur  or 
humerus  saw  a  disk  about  one-quarter  of  an  inch  thick.  In 
doing  this  notice  the  appearance  of  the  marrow  cavity 
especially  as  you  look  into  it  toward  the  articular  ends. 
Saw  the  disk  into  sectors  with  an  arc  of  about  a  quarter  of 
an  inch  on  the  outer  surface.  From  this  piece  saw  two  thin 
sHces — one  at  right  angles  to  the  axis  of  the  bone,  the 
other  parallel  with  it.  These  should  be  ground  as  directed 
in  the  introduction  for  the  grinding  of  transverse  sections 
of  the  root,  and  be  brought  to  the  laboratory  ready  to  mount. 
They  should  be  mounted  in  hard  balsam  as  described  in 
the  mounting  of  longitudinal  sections  of  the  teeth.  Label 
the  slide  with  the  name  of  the  bone  from  which  the  section 
is  taken  and  the  direction  in  which  it  is  cut.  Study  the 
transverse  section  with  the  low  power,  working  out  the 


442  DIRECTIONS  FOR  LABORATORY  WORK 

arrangement  of  the  lamellae  and  the  distribution  of  the 
subperiosteal  and  Haversian  system  bone  (p.  252).  Draw 
the  tissue  from  the  surface  of  the  bone  to  the  marrow  cavity. 
This  drawing  should  be  not  more  than  an  inch  wide  and  the 
full  length  of  the  page.  With  the  high  power  objective  and 
low  power  eyepiece,  draw  one  or  two  Haversian  systems. 

Study  the  arrangement  of  the  Haversian  canals  as  seen 
in  the  longitudinal  sections.  With  the  high  power  draw 
at  least  three  lacunae,  showing  one  cut  lengthwise,  one  trans- 
versely, and  one  as  seen  from  above. 


PERIOD  XI 

Decalcified  Bone. — One  of  the  bones  from  a  small  animal 
has  been  decalcified,  embedded,  sectioned,  and  stained  with 
hematoxylin  and  eosin.  Receive  from  the  desk  two  sections, 
one  of  which  is  cut  longitudinally,  the  other  transversely. 
Mount  in  balsam  in  the  usual  way.  Label  the  slide  with 
the  name  of  the  animal,  the  bone  from  which  it  is  cut,  and 
the  direction  of  the  section.  Study  the  transverse  section 
with  the  low  power,  noting  the  bone  corpuscles  in  the  lacunae, 
the  tissue  in  the  Haversian  canals,  and  the  marrow.  With 
the  high  power  draw  one  field  showing  two  or  three  Haversian 
systems,  one  of  which  has  been  partially  destroyed  in  the 
building  of  another.  Draw  with  the  high  power  one  field 
from  the  marrow  cavity.  From  the  longitudinal  section 
draw,  with  the  high  power,  one  field  showing  osteoblasts  in 
a  medullary  space. 

PERIOD  XII 

Comparative  Study  of  Subperiosteal  Bone  and  Cementum. — 
For  this  day's  work  the  previously  mounted  sections  must 
be  used,  the  longitudinal  sections  of  the  teeth,  the  transverse 
sections  of  the  root,  the  ground  and  decalcified  sections  of 
bone.  Study  the  cementum  and  the  subperiosteal  bone  as 
shown  in  these  sections  and  make  one  drawing  of  cementum 


DENTAL  PULP  FROM  UNERUPTED  TOOTH  OF  SHEEP  443 

and  one  drawing  of  subperiosteal  bone  to  show  the  com- 
parison in  structure.  Compare  the  regularity  in  form  and 
arrangement  of  the  lacunae  in  the  bone  with  the  irregularity 
in  form  and  position  of  the  lacunae  in  cementum.  Note  that 
in  the  bone  the  lacunae  lie  between  the  layers ;  in  the  cementum 
they  may  be  between  the  layers  or  entirely  within  a  single 
layer.  Compare  the  regularity  in  the  arrangement  and  thick- 
ness of  layers  with  the  corresponding  irregularity  in  cementum. 
Note  the  size,  number,  and  arrangement  of  the  canaliculi 
radiating  from  the  lacunae  in  bone,  and  compare  them  with 
the  canaliculi  of  the  cementum. 


PERIOD  xm 

Dental  Pulp  from  the  Unerupted  Tooth  of  a  Sheep. — An 

unerupted  molar  or  premolar  of  a  yearling  lamb  was  removed 
from  the  lower  jaw  by  splitting  the  bone.  The  pulp  was 
pulled  out  of  the  partially  formed  dentine  embedded  in 
paraffin,  sectioned,  stained  with  hematoxylin  and  eosin. 
Bring  to  the  desk  a  clean  slide  with  a  drop  of  balsam  upon 
the  centre  of  it  and  receive  a  section.  Label  the  slide:  " Pulp 
from  unerupted  tooth  of  sheep,  stained  with  hematoxylin 
and  eosin."  Study  first  with  the  low  power.  Upon  the 
circumference  of  the  section  the  layer  of  odontoblasts  may 
or  may  not  be  shown,  depending  upon  whether  in  the 
removal  of  the  pulp  the  fibrils  have  pulled  away  from  the 
dentine,  or  the  odontoblasts  have  been  pulled  oft'  from  the 
surface  of  the  pulp.  They  are  usually  present,  at  least  in 
spots.  Note  the  number  and  arrangement  of  the  blood- 
vessels and  the  distribution  of  the  connective-tissue  cells. 
With  the  low  power  draw  a  portion  from  the  surface  to  the 
centre,  showing  the  layer  of  odontoblasts  if  present.  With 
the  high  power  draw  one  field  showing  a  bloodvessel  and 
the  connective-tissue  cells,  taking  particular  pains  to  repre- 
sent their  forms  correctly.  If  there  are  any  odontoblasts 
present  draw  one  field  showing  them  and  the  layer  of  Weil 
(see  page  209). 


444  DIRECTIONS  FOR  LABORATORY  WORK 


PERIOD  XIV 

Dental  Pulp,  Normal  Human. — A  number  of  human  teeth 
were  cracked  immediately  after  extraction  and  the  pulps 
removed  from  the  pulp  chambers.  They  were  embedded  in 
one  block  of  paraffin,  sectioned,  stained  with  hematoxylin 
and  eosin,  and  are  ready  to  be  given  out.  Bring  to  the  desk 
a  clean  slide  with  a  drop  of  balsam  on  the  centre  and  receive 
a  section.  Label  the  slide:  "Transverse  section  of  pulp  from 
human  teeth."  There  will  be  several  sections  in  this  specimen, 
each  from  a  separate  pulp.  With  the  low  power  follow  the 
circumference  of  each  section,  looking  for  places  where 
odontoblasts  are  present.  Find  the  best  field  in  the  speci- 
men and  draw  the  layer  of  odontoblasts  as  seen  with  the 
high  power.  Notice  the  fibrils  which  have  been  pulled  out 
of  the  dentinal  tubules  projecting  from  the  ends  of  the 
odontoblasts.  If  the  section  is  parallel  with  the  long  axis 
of  the  cells,  they  will  appear  as  tall  columnar  cells  with  a 
nucleus  in  the  deeper  end.  If  it  is  oblique  to  their  axis  the 
layer  may  appear  as  two  or  three  layers  of  oval  cells.  Just 
beyond  the  odontoblasts  the  layer  of  Weil  Avill  be  seen, 
usually  appearing  as  a  clearer  layer  containing  few  cells  and 
about  half  as  wide  as  the  odontoblasts.  Beyond  this  the 
connective-tissue  cells  are  thickly  placed  for  a  short  distance, 
and  still  deeper  they  are  more  widely  scattered  and  about 
evenly  distributed  in  the  rest  of  the  pulp. 

With  the  high  power  draw  one  field  to  show  the  form  of 
the  connective-tissue  cells  of  the  pulp.  With  the  low  power 
study  the  distribution  of  the  bloodvessels  in  all  of  the  sec- 
tions. Select  the  best  section  and  draw  the  entire  section, 
to  show  the  size,  number,  and  arrangement  of  the  large 
bloodvessels.  With  the  high  power  draw  a  single  field,  to 
show  accurately  the  structure  of  a  bloodvessel  wall. 


ENDOCHONDRIAL  BONE  FORMATION  445 


PERIOD  XV 

Dental  Pulp,  Pathologic  Human. — By  the  cooperation  of 
the  man  in  charge  of  the  extracting  room,  or  an  extracting 
speciahst,  teeth  with  hving  but  inflamed  or  h\7)eremic 
pulps  were  dropped  as  soon  as  extracted  into  a  fixing  fluid. 
The  teeth  were  afterward  cracked  and  the  pulps  removed, 
embedded,  and  sectioned  as  before.  Bring  to  the  desk  a 
clean  slide  with  a  drop  of  balsam  on  its  centre  and  receive  a 
section.  Label  the  sHde:  "Pathologic  pulp  from  human 
tooth  stained  T^'ith  hematoxylin  and  eosin."  Follow  the 
same  routine  in  studying  these  specimens  as  in  the  case  of 
the  normal  pulp.  It  is  impossible  to  tell  just  what  condi- 
tions will  be  present.  Compare  the  size  and  number  of  the 
bloodvessels  with  those  in  the  normal  tissue,  and  the  char- 
acter and  distribution  of  the  cellular  elements.  Look  for 
nodules  of  calcoglobuU,  especially  in  the  inflammatory  speci- 
mens, and  make  a  diagnosis  of  the  condition,  as  shown  in 
the  specimen.  See  the  chapter  on  the  Structural  Changes  in 
the  Pulp  and  Pathological  Conditions  for  fuither  assistance 
on  the  work  in  this  material. 


PERIOD  XVI 

Endochondrial  Bone  Formation. — A  forming  bone  from  a 
human  fetus  has  been  embedded,  sectioned,  and  stained  with 
hematoxylin  and  eosin.  Receive  a  section  from  the  desk  and 
mount  as  usual.  Study  the  specimen  -^dth  the  low  power, 
identifying  first  the  general  arrangement  of  the  tissues, 
following  from  the  unchanged  cartilage  to  the  development 
of  bone.  Notice  the  subperiosteal  layers  on  the  surface. 
Make  a  sketch  of  a  sufficient  part  of  the  section  to  show  the 
changes  from  the  typical  hyaline  cartilage  to  the  young  bone. 
With  the  high  power  draw  one  field  from  a  primary  marrow 
cavity,  showdng  osteoblasts  laying  down  lamellse  on  one  of  the 
spicules,  and  one  field  showing  osteoclasts. 


446  DIRECTIONS  FOR  LABORATORY  WORK 


PERIOD  xvn 

Bone  Growth. — A  piece  of  a  long  bone  from  a  very  young 
animal  has  been  embedded  and  sectioned  transversely  to  the 
shaft.  Sections  have  been  stained  in  hematoxylin  and 
eosin,  to  be  mounted  as  usual.  Label  the  slide:  "Growing 
bone  cut  transversely,  stained  with  hematoxylin  and  eosin." 
Study  first  with  the  low  power.  On  the  surface  of  the  section 
will  be  seen  the  periosteum,  in  which  the  fibrous  and  osteo- 
genetic  layers  can  be  easily  recognized.  Bone  formation  is 
actively  going  on,  laying  down  lamella^  under  the  periosteum 
which  are  being  transformed  into  Haversian  system  bone. 
With  the  low  power  draw  a  portion  of  the  section  from  the 
periosteum  to  the  centre  of  the  bone.  With  the  high 
power  draw  a  field  showing  the  osteoblasts  of  the  peri- 
osteum, a  field  showing  the  absorption  of  subperiosteal  bone 
to  form  a  medullary  space,  and  a  field  showing  osteoblasts  in 
a  medullary  space. 

PERIOD  XVIII 

Periosteum  from  Attached  Portion. — From  a  young  kitten 
a  portion  of  a  bone  in  a  region  to  which  muscles  are  attached 
to  the  periosteum  was  carefully  dissected  out,  removing  the 
attached  muscle,  and  the  tissue  embedded  in  celloidin,  the 
sections  cut  parallel  to  the  axis  of  the  bone  and  perpendicular 
to  its  surface.  They  have  been  stained  in  hematoxylin  and 
eosin,  and  are  ready  to  be  given  out.  Receive  a  section  and 
mount  as  usual.  Label :  "  Periosteum  from  attached  portion, 
stained  in  hematoxylin  and  eosin."  Study  the  specimen  first 
with  the  low  power.  The  outer  fibrous  layer  of  the  peri- 
osteum will  be  seen  with  the  muscle  fibers  attached  to  it 
and  the  osteogenetic  layer  with  the  greater  number  of  cells 
taking  the  stain  more  deeply.  Draw  with  the  low  power, 
showing  the  tissues  from  the  surface  of  the  periosteum  well 
into  the  substance  of  the  bone.  With  the  high  power  study 
the  attachment  of  the  muscle  fibers  to  the  outer  layer  of  the 
periosteum,  the  character  and  arrangement  of  the  fibers  of 


GINGIVUS  AND  GUM  TISSUE  447 

the  outer  layer,  the  interlacing  of  the  fibers  of  the  outer 
and  inner  layer,  the  cells,  and  especially  the  osteoblasts  of 
the  inner  layer  and  the  penetrating  fibers  that  are  built  into 
the  bone.  Draw  the  thickness  of  the  periosteum  as  seen 
with  the  high  power,  showing  the  details  of  structure  as 
accurately  as  possible. 

PERIOD  XJX 

Gingivus  and  Gum  Tissue. — The  gingivus  and  gum  tissue 
covering  the  alveolar  process  down  to  the  point  of  reflection 
on  to  the  cheek  was  dissected  away  from  the  teeth  and  jaw 
of  a  sheep.  The  tissue  was  embedded  in  paraffin  and  sec- 
tioned parallel  \^'ith  the  long  axis  of  the  tooth.  The  sections 
have  been  stained  with  hematoxylin  and  von  Gieson,  and  are 
ready  to  mount.  Bring  to  the  desk  a  clean  slide  ^^'ith  a  drop 
of  balsam  on  the  centre  and  receive  a  specimen.  Label  the 
section:  "Gingivus  from  a  sheep,  stained  with  hematoxylin 
and  von  Gieson."  By  this  staining  the  cellular  elements 
will  have  a  brownish  color,  the  nuclei  dark,  the  protoplasm 
lighter,  the  white  fibers  should  be  bright  red,  and  the  elastic 
fibers  yellowish.  It  is  a  specially  good  stain  for  connective 
tissue.  Study  with  the  low  power.  The  epithelial  will  be 
stained  a  brownish  yellow  or  purple.  It  is  a  stratified 
squamous  epithelium  made  up  of  many  layers  of  cells  and 
with  a  distinct  horny  or  corneous  layer  on  the  surface  from 
the  crest  of  the  gingivus  to  the  point  where  the  mucous  mem- 
brane is  reflected  on  to  the  cheek,  or  where  it  ceases  to  be 
attached  to  the  gum.  This  layer  is  yello\\Tsh  in  color,  and  is 
made  up  of  closely  packed  scales  having  no  nuclei.  They  are 
the  remains  of  epithehal  cells  from  which  the  protoplasm  is 
gone,  leaving  only  the  horny  material  which  it  had  produced. 
The  portion  of  the  epithelial  lining  the  gingival  space  has  no 
corneous  layer,  nuclei  being  seen  in  the  cells  at  the  surface. 
The  cells  are  larger  and  more  loosely  placed.  The  connective- 
tissue  papillffi  and  the  projections  of  epithelium  which  are 
between  them  are  extremely  long.  In  the  epithelium 
covering  the  alveolar  process  the  connective-tissue  papilla? 


448  DIRECTIONS  FOR  LABORATORY  WORK 

are  broader  and  not  so  deep,  and  the  cells  are  much  more 
compactly  arranged.  At  the  point  of  reflection  on  the  cheek 
the  epithelium  changes  its  character  abruptly,  the  corneous 
layer  disappears,  the  surface  cells  showing  nuclei,  the  epi- 
thelial layer  is  thicker  and  made  up  of  larger  and  more 
loosely  placed  cells.  This  change  in  the  structure  explains 
why  the  epithelium  is  easily  broken  where  a  movable  portion 
of  the  membrane  passes  over  the  edge  of  an  artificial  denture. 
When  an  infection  reaches  the  connective  tissue  a  sore  is 
produced  that  requires  some  time  to  heal. 

Study  the  connective  tissue,  which  is  made  up  of  coarse, 
wavy  bundles  of  white  fibers  taking  the  red  stain.  In  the  gum 
tissue,  that  is,  the  portion  of  the  section  covering  the  alveolar 
process,  the  bundles  are  very  large  and  form  a  very  coarse 
network.  Beyond  the  point  of  reflection  the  bundles  are 
finer  and  more  delicate  in  their  arrangement.  Elastic  fibers 
take  the  yellowish  stain.  Notice  the  bloodvessels  in  the 
connective  tissue  and  the  capillaries  in  the  papillae.  With 
the  low  power  draw  the  entire  section  so  as  to  show  the 
character  of  the  epithelium  and  the  fibrous  tissue  in  the  three 
parts. 

With  the  high  power  draw  the  thickness  of  the  epithelium 
lining  the  gingival  space  and  at  the  point  where  the  mem- 
brane is  reflected  to  the  cheek. 

PERIOD  XX 

Peridental  Membrane,  Transverse  Gingival. — The  lower  jaw 
of  a  young  sheep  was  sawed  through  between  the  teeth, 
cutting  the  jaw  into  blocks  each  containing  two  teeth.  The 
crowns  were  broken  off  or  opened  so  as  to  admit  the  fluids 
to  the  pulp  tissue.  The  tissues  were  decalcified,  embedded, 
and  sectioned  at  right  angles  to  the  axis  of  the  tooth.  They 
are  cut  from  the  gingival  portion,  and  have  been  stained  with 
hematoxylin  and  eosin.  Receive  a  section  and  mount  as 
usual.  Label  the  slide :  "  Peridental  membrane,  transverse 
gingival,  stained  with  hematoxylin  and  eosin.''  A  similar 
block  of  tissue  preserved  in  alcohol  will  be  found  at  the  desk. 


PERIDENTAL  MEMBRANE  449 

This  should  be  observed  so  as  to  study  out  the  relation 
of  the  section  to  the  gross  appearance  of  the  tissue. 

Holding  the  section  to  the  light,  observe  the  distribution 
of  the  tissue.  Two  roots  '^'ill  be  seen  cut  across.  Observe  the 
epithelium  on  the  labial  and  the  lingual,  and  possibly  also  that 
lining  the  gingival  space  lying  next  to  the  root  of  one  of  the 
teeth.  By  the  aid  of  the  low  power  sketch  the  outline  of  the 
entire  section  to  show  the  distribution  of  the  tissues.  Note 
the  demarcation  where  the  finer  fibers  of  the  peridental 
membrane  unite  with  the  coarser  mat  of  gum  tissue.  Begin- 
ning at  the  centre  of  the  labial  surface,  follow  the  fibers 
springing  from  the  cementum  to  where  they  are  lost  in  the 
gum  tissue  or  attached  to  the  approximating  tooth.  Draw 
the  portion  of  the  membrane  between  the  two  roots,  accu- 
rately representing  the  arrangement  of  the  fibers.  The  epi- 
thelial structures  wall  be  seen  lying  between  the  fibers  close  to 
the  cementum,  and  should  be  showm  in  the  drawing  (p.  308). 

With  the  high  power  study  the  cementoblasts  and  the 
epithelial  structures.  Make  a  dra-^ing  of  one  field,  showing 
all  the  details  of  structure  as  accurately  as  possible. 

With  the  high  power  draw  one  field  showing  the  fibrous 
tissue  between  the  roots  and  the  relation  of  the  fibroblasts 
to  them.    This  field  should  include  a  bloodvessel. 

PERIOD  XXI 

Peridental  Membrane,  Alveolar  Portion,  Transverse. — The 
sections  for  this  work  have  been  cut  from  the  same  block  as 
the  preceding,  but  are  in  the  occlusal  third  of  the  alveolar 
portion  and  as  close  to  the  border  of  the  alveolar  process 
as  possible.  Receive  a  section.  Mount  as  usual  and  label 
the  shde :  "  Peridental  membrane,  alveolar  portion,  transverse, 
stained  with  hematoxylin  and  eosin." 

Study  the  general  arrangement  of  the  tissues  and  make  a 
sketch  as  in  the  case  of  the  previous  specimen.  Note  the 
muscle  fibers  from  the  muscles  of  the  lip  attached  to  the 
periosteum  on  the  labial  surface  of  the  process,  the  bone  of 
the  labial  plate,  the  septum  separating  the  alveoli,  the  peri- 
20 


450  DIRECTIONS  FOR  LABORATORY  WORK 

dental  membrane  filling  the  space  between  the  bone  and  the 
surface  of  the  root,  the  layers  of  the  cementum,  the  dentine 
and  the  pulp. 

After  studying  the  specimen  with  the  low  power  as  care- 
fully as  possible,  draw  the  peridental  membrane  surrounding 
one  root,  including  the  thickness  of  the  labial  plate  of  bone 
with  its  periosteum  and  a  part  of  the  lingual  plate.  In  this 
drawing  represent  accurately  the  fibers  of  the  peridental 
membrane,  their  arrangement  in  the  bundles,  and  the  relation 
of  the  bundles  to  each  other  and  the  bloodvessels.  To  do 
this  the  fine  adjustment  must  be  used  to  obtain  ideas  of  the 
third  dimension  of  space.  With  the  high  power  draw  one 
field  from  the  wall  of  the  alveolus,  showing  the  attachment  of 
the  fibers  to  the  bone,  the  osteoblasts  on  the  surface  of  the 
bone,  and  the  other  cellular  elements.  This  field  should 
include  a  bloodvessel.  With  the  high  power  draw  the  thick- 
ness of  the  cementum  at  some  point  where  a  specially  strong 
bundle  of  fibers  is  attached.  This  should  show  the  fibers 
embedded  in  the  cementum,  cementoblasts  on  the  surface, 
and  the  branching  and  interlacing  of  the  bundles. 

PERIOD  xxn 

Longitudinal    Section    of    the   Peridental   Membrane. — The 

lower  incisor  of  a  young  sheep  was  removed  from  the  jaw 
by  sawing  through  between  the  teeth,  leaving  two  teeth  in 
each  block.  The  crowns  of  the  teeth  were  broken  off  near 
the  level  of  the  gum  so  as  to  admit  the  reagents  to  the  pulp 
chamber.  The  tissues  decalcified,  embedded  in  celloidin, 
and  sectioned.  They  were  cut  through  from  labial  to 
lingual,  and  only  the  ones  from  the  central  portion  used. 
They  have  been  stained  in  hematoxylin  and  eosin  and  are 
ready  to  mount.  Mount  the  section  as  usual  and  label  the 
slide :  "  Longitudinal  section  through  the  peridental  membrane 
of  a  sheep,  labiolingual,  stained  in  hematoxylin  and  eosin." 
First  hold  the  section  up  to  the  light  and  note  the  relation 
of  the  tooth  to  the  bone  and  the  soft  tissues.  Study  the 
section  with  the  low  power  and  make  a  sketch  showing  the 


TOOTH  GERM  451 

general  distribution  of  the  tissues.  Show  the  pulp  chamber, 
dentine  and  cementum,  bone,  periosteum,  gum  tissue,  and 
epithelium.  Do  not  attempt  to  fill  in  the  drawing  more  than 
diagrammatically,  for  it  would  require  too  much  time.  The 
object  of  the  drawing  is  to  get  the  general  relation  of  the 
tissue  before  studying  parts  of  it  in  detail.  Compare  the 
form  of  the  labial  and  the  lingual  gingivus  and  make  a  draw- 
ing of  the  lingual,  showing  the  details  of  structure  as  far  as  the 
border  of  the  process  and  as  accuratel}^  as  possible.  With 
the  high  power  draw  the  thickness  of  the  epithelium  lining 
the  gingival  space.  Study  the  fibers  in  the  occlusal  third 
of  the  alveolar  process  and  make  a  drawing  to  represent  them 
accurately,  showing  the  cementum  at  one  side  and  the  bone 
at  the  other.  The  entire  length  of  the  root  can  seldom  be 
got  in  one  section  on  account  of  the  curve  of  the  tooth, 
so  that  the  fibers  can  probably  be  studied  to  advantage  in 
the  occlusal  third  of  the  alveolar  process  only.  Draw  one 
field  with  the  high  power  showing  the  bloodvessels. 

PERIOD  xxm 

Tooth  Germ. — The  head  of  an  embryo  pig  was  embedded 
in  paraffin  and  sectioned  at  right  angles  to  the  snout.  The 
sections  begin  in  the  region  of  the  incisors  and  far  enough 
back  to  cut  through  the  nose  cavity.  They  have  been 
stained  in  hematoxylin  and  eosin.  Bring  to  the  desk  a  clean 
sHde  and  receive  a  section.  Label  the  shde:  "Tooth  germ, 
stained  with  hematoxylin  and  eosin." 

The  general  form  of  the  section  will  depend  on  the  position 
of  the  section  through  the  head.  At  the  desk  is  the  head 
of  a  similar  embrj'o  preserved  in  alcohol.  This  should  be 
observed  so  as  to  determine  from  the  section  its  relation  to 
the  head.  By  holding  the  section  to  the  light  and  the  use 
of  the  low  power,  make  a  sketch  of  the  entire  section.  Note 
the  epiblast  covering  the  outer  surface  and  lining  the  nose 
and  mouth  cavity.  The  mass  which  is  to  form  the  tongue 
lying  between  the  roof  of  the  mouth  and  the  mandibular 
arch.    If  the  section  is  in  front  of  the  angle  of  the  mouth 


452  DIRECTIONS  FOR  LABORATORY  WORK 

there  will  be  no  connection  between  the  upper  and  lower 
parts  of  the  section.  Notice  the  separation  of  the  nose 
cavity  into  right  and  left  by  a  septum  containing  cartilage, 
and  the  projections  of  cartilage  from  the  side  walls  which 
will  form  the  turbinate  bones.  On  either  side  of  the  septum 
where  it  joins  the  palate  will  be  seen  little  structures  known 
as  Jacobson's  organ,  which  later  disappear.  Notice  Meckel's 
cartilage  in  the  mesodermic  mass  of  the  mandible.  In  the 
epiderm  of  the  outer  surface  the  beginning  of  the  formation 
of  hairs  are  to  be  seen. 

With  the  low  power  follow  the  epiderm  lining  the  mouth 
cavity  and  look  for  the  tooth  germ.  In  each  section  there 
are  four  chances  for  tooth  germs,  one  on  either  side  in  the 
upper  and  lower  arches.  Select  the  best  one  and  draw  it 
as  seen  with  the  low  power.  The  appearance  will  depend 
entirely  upon  the  stage  of  development. 

With  the  high  power  draw  enough  of  the  enamel  organ 
to  show  the  arrangement  of  the  cells  in  the  outer  and  inner 
tunics  and  the  stellate  reticulum. 


PERIOD  XXIV 

Tooth  Germ. — Sections  have  been  prepared  in  the  same  way 
as  in  the  preceding,  but  from  the  head  of  an  older  embryo, 
in  which  the  tooth  germs  are  completely  formed  and  calci- 
fication is  ready  to  begin. 

Receive  a  section,  mount,  and  label  as  before,  and  draw  the 
outline  of  the  entire  section.  Note  the  changes  in  form  and 
in  the  tissue  elements  from  the  previous  section.  Bone 
formation  has  begun  both  in  the  mandible  and  the  maxilla. 
The  amount  and  distribution  of  this  should  be  carefully 
studied. 

With  the  low  power  draw  the  entire  tooth  germ,  selecting 
the  most  typical  one  in  the  section.  With  the  high  power 
draw  one  field  showing  ameloblasts,  odontoblasts,  and  a 
portion  of  the  papillae.  Find  a  field  in  which  bone  formation 
is  going  on  and  draw  it  accurately  with  the  high  power. 


APPENDIX  CHAPTEPv  I 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS,  USING 
THE  GRINDING  MACHINE 

By  G.  V.  BLACK,  M.D.,  D.D.S.,  Sc.D.,  LL.D. 

The  Machine. — The  basis  of  this  machine  is  the  larger 
watchmaker's  lathe  known  as  No.  2.  It  must  swing  4 
inches,  the  length  of  the  bed  must  be  12  inches,  and  be  good 
and  solid.  A  test  should  be  made  of  the  alignment  of  the 
lathe  head  to  see  that  this  is  exact.  If  there  is  any  inac- 
curacy, another  lathe  should  be  selected.  The  power  should 
consist  of  one  of  the  largest  and  strongest  electric  lathes, 
or  motors,  made  for  the  use  of  dentists.  This  power  should 
be  transmitted  to  the  lathe  through  an  overhead  shaft  of 
a  length  that  will  give  good  room  to  operate  the  lathe  with- 
out the  motor  being  in  the  way.  A  pulley  may  be  placed  on 
the  left  end  of  the  shaft  of  the  motor  on  one  of  the  brass 
carriers  for  grinding  wheels.  This  pulley  should  carry  a 
good  quarter-inch  round  leather  belt.  Its  diameter  should 
be  2|  inches.  The  pulley  on  the  right  hand  end  of  the  shaft 
above  should  be  5  inches.  This  will  reduce  the  speed  one- 
half  and  double  the  power.  On  the  left  end  of  the  shaft 
should  be  placed  a  copy — reversed — of  the  pulley  on  the 
lathe-head,  which  has  4  grooves.  This  gives  good  varieties  of 
speed  with  each  speed  of  the  motor.  Another  small  pulley 
will  be  placed  near  the  centre  of  the  length  of  the  overhead 
shaft,  the  purpose  of  which  will  be  explained  later  (Figs. 
343  and  344). 

The  grinding  apparatus  is  built  upon  a  base  fitted  to  the 
lathe  bed  in  the  same  wav  as  the  lathe  head,  or  tailpiece. 


454 


APPENDIX  CHAPTER  I 

Fig.  343 


Figs.  343  and  344. — A  general  view  of  the  grinding  machine,  showing  particularly 
the  arrangement  for  transmitting  the  power  from  the  electric  motor  to  the  machine  that 
does  the  work.  All  of  this  may  be  made  out  by  reference  to  the  picture  while  following 
the  text.  The  bed  of  the  little  lathe  on  the  left  hand  is  \2\  inches  long,  which  gives 
a  good  idea  of  the  general  dimensions. 

The  water  is  delivered  to  the  grinding  stone  from  a  rubber  bag  or  bucket  hung  on 
the  frame  above  through  a  rubber  tube  to  the  metal  tube  on  a  movable  stand,  which 
may  be  so  placed  as  to  bring  the  brush  at  its  end  against  the  stone.  This  stand  and 
brush  are  better  seen  in  Fig.  344. 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS     455 


Fig   344 


456  APPENDIX  CHAPTER  I 

It  has  one  main  shaft  parallel  with  the  lathe  bed,  in  good 
and  sufficient  bearings  to  maintain  accuracy  of  alignment 
and  perfect  steadiness  for  long  continued  usage  (see  Figs.  343 
and  344).  This  shaft  moves  freely  lengthwise,  or  back  and 
forward,  while  turning  slowly  in  its  bearings.  On  the  end 
of  this  shaft  next  to  the  lathe  head — the  forward  end — 
there  is  a  larger  portion,,  or  ring,  and  this  end  terminates 
in  a  threaded  nipple,  upon  which  the  removable  grinding 
disks  are  screwed  firmly  against  the  face  of  this  larger  ring, 
to  secure  accuracy  of  adjustment.  The  use  of  these  disks 
will  be  more  fully  explained  later. 

On  the  rear  end  of  this  shaft,  just  back  of  its  rear  bearing 
and  abutting  against  it,  a  large  movable  nut  is  placed.  This 
is  provided  with  a  thumb  screw  by  which  it  is  made  fast 
at  any  point  desired.  Turning  this  forward  pulls  the  shaft 
back  from  the  grinding  stone.  Turning  it  backward  allows 
the  shaft  to  move  forward  against  the  stone.  It  has  also 
a  finger  reaching  back  over  a  graduated  disk  just  to  its 
rear.  This  disk  is  made  fast  on  the  shaft,  and  the  two 
together  constitute  the  micrometer,  by  which  the  thickness 
to  which  specimens  are  ground  is  measured.  The  movable 
nut  has  40  threads  to  the  inch.  The  graduation  of  the  disk 
is  on  the  same  principle  as  that  on  the  screw  calipers  used 
by  machinists  for  fine  measurements — one-thousandth  of  an 
inch — but  as  this  disk  is  li  inches  in  diameter,  the  gradu- 
ations of  thousandths  are  so  wide  that  one-quarter  of  one- 
thousandth  may  readily  be  used.  It  differs  in  plan,  in 
that  both  the  graduation  and  the  parallel  lines  are  placed 
upon  this  disk.  On  the  machinist's  micrometer  the  lines 
are  placed  on  the  shaft  and  the  graduations  on  the  nut. 
The  graduation  is  read  from  the  side  of  the  finger  on  the 
movable  nut,  and  the  lines  are  read  from  its  end.  It  is  a 
very  perfect  micrometer  (Figs.  345  and  346). 

The  forw^ard  movement  of  the  shaft  when  grinding,  and 
also  the  pressure  exerted  upon  the  stone,  are  furnished  by  a 
tailpiece  placed  behind  it  and  attached  to  the  lathe  bed. 
This  has  a  plunger  actuated  by  a  spiral  spring,  which  pushes 
the  shaft  forward  against  the  stone.    The  amount  of  pressure 


THE  GRINDIXG  OF  MICROSCOPIC  SPECIMENS     457 

exerted  in  the  grinding  is  controlled  by  the  amount  of  com- 
pression of  this  spring  in  fixing  the  piece  to  the  lathe  bed. 
It  may  be  much  or  little,  as  desired.  Usually  very  little 
pressure  is  used.  When  the  movable  nut  has  come  against 
the  frame  in  which  this  shaft  turns,  the  machine  may  con- 
tinue to  run,  but  the  forward  movement  of  the  shaft  stops 
and  the  grinding  ceases  in  consequence.  Therefore  there  is 
no  danger  of  grinding  a  specimen  thinner  than  the  measure- 
ment fixed  upon.  The  further  arrangement  for  finding  this 
measurement  will  be  described  later. 

On  the  rear  portion  of  the  graduated  disk,  or  wheel,  a 
portion  or  space  is  toothed,  and  connected  with  a  worm 
pinion  or  threaded  shaft  by  which  the  main  shaft  is  turned 
in  its  bearings.  A  belt  is  attached  over  a  wheel  on  the  end 
of  this  worm  shaft,  and  extends  to  the  third  wheel,  previously 
mentioned,  on  the  overhead  shaft.  When  this  belt  is  ad- 
justed and  the  motor  started,  it  causes  the  main  shaft  in 
the  grinding  machine  proper  to  turn  slowly  on  its  axis,  while 
being  pressed  against  the  stone  by  the  tailpiece.  By  this 
arrangement  every  part  of  the  specimen  fixed  on  the  grind- 
ing disk  is  brought  successively  against  every  part  of  the 
rapidly  revolving  stone,  and  is  cut  perfectly  level  in  all  of 
its  parts. 

The  Grinding  Disks. — The  grinding  disks  are  of  brass, 
accurately  turned  |  inch  thick,  and  If  inches  in  diameter. 
They  have  a  threaded  hole  i  inch  deep  in  the  back  to  fix 
them  to  the  nipple  on  the  forward  end  of  the  shaft  of  the 
grinding  machine.  A  machine  should  have  a  half-dozen 
or  more  of  these,  lettered  or  numbered  on  the  edge,  so  that 
records  of  each  may  be  made  when  measuring  preparatory 
to  mounting  specimens  for  grinding.  As  the  mounting  of 
specimens  on  others  of  these  may  proceed  while  the  grinding 
on  one  is  going  on  (for  the  machine,  being  automatic,  needs 
little  attention),  this  number  at  the  least  is  necessar}^  for 
rapid  work. 

The  machine  may  be  stopped  and  the  disk  removed  from 
the  shaft  by  a  few  backward  turns,  the  progress  of  the 
grinding  examined,  the  disk  returned  for  further  grinding, 


458 


APPENDIX  CHAPTER  I 

Fig.  345 


Figs.  345  and  346. —  The  lathe  with  the  grinding  machine  mounted  upon  it  in  position  for  work.  On  the  left 
next  to  the  lathe  head  is  the  grinding  stone  surrounded  by  the  spatter  guard,  which  gathers  all  of  the  water 
from  the  wheel  and  dehvers  it  through  its  hollow  post  into  a  rubber  tube  below  the  lathe  bed,  which  conveys  it 
to  a  conveniently  placed  receptacle.  The  water  comes  from  a  rubber  bag  or  bucket  hung  on  the  overhead 
frame  (see  Fig.  343)  through  a  rubber  tube  to  the  metal  tube  mounted  on  a  movable  stand  so  that  the 
brush  through  which  it  passes  may  be  placed  against  the  stone.  The  grinding  machine  proper  is  secured 
to  the  lathe  bed  by  the  larger  thumbscrew  seen  below.  The  point-finder  is  seen  at  the  foot  of  the  spatter 
guard,  and  is  secured  by  the  middle  thumbscrew  seen  below  the  lathe  bed. 

The  shaft  of  the  grinding  machine  (6  inches  long)  runs  through  its  whole  length,  but  is  completely 
covered  in  by  its  housings  to  protect  its  bearings  from  grit,  except  at  its  forward  end  (next  to  the  grinding 
stone).  This  part  is  protected  by  a  swaddle  held  by  a  ring,  which  keeps  the  working  bearing  clean.  On 
this  end  the  grinding  disk  is  seen  almost  touching  the  stone.  The  micrometer  is  on  the  other  end  of  the 
shaft  next  back  of  the  frame  of  the  grinding  machine.  Next  back  of  this  is  a  toothed  wheel  made  fast  to 
the  shaft.  This  is  actuated  by  the  middle  one  of  the  belts  descending  from  overhead  (Fig.  343,  the  left 
hand  belt  in  Fig.  344).  This  belt  passes  over  a  wheel  hidden  from  view  and  through  a  small  worm  shaft  turns 
the  main  shaft.  Pressure  for  the  grinding  is  supplied  by  a  plunger  actuated  by  a  spiral  spring  seen  at  the 
extreme  right  hand  end. 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS     459 


460  APPENDIX  CHAPTER  I 

etc.,  at  any  time  during  the  progress  of  the  work.  The  face 
of  the  disk,  which  should  be  perfectly  flat  and  parallel  with 
the  face  of  the  stone,  should  always  be  perfectly  bright, 
so  as  to  reflect  light  through  the  specimen  when  it  becomes 
thin.  This  enables  one  to  judge  very  closely  of  the  thickness 
by  the  eye  (after  sufficient  practice),  that  sometimes  proves 
a  valuable  check  on  the  setting  of  the  measurement  in  the 
beginning. 

The  Point  Finder. — This  is  a  piece  of  steel  one-eighth  of 
an  inch  thick,  fitted  to  the  lathe  bed  and  set  against  the  face 
of  the  lathe  head,  and  made  fast  by  a  thumb  screw  passing 
through  the  lathe  bed  from  below.  It  has  a  strong  arm  which 
passes  around  other  fixtures  between  the  lathe  head  and  the 
forward  end  of  the  base  of  the  grinding  machine.  It  is  pro- 
vided with  a  set  screw,  by  which  a  range  of  variation  can  be 
made  in  the  distance  of  the  forward  end  of  the  frame  of 
the  grinding  machine  from  the  lathe  head.  When  this  is 
in  place  and  the  measurement  of  a  disk  has  been  made  and 
recorded  for  the  grinding  of  a  specimen  to  a  specified  thick- 
ness, the  machine  may  be  taken  to  pieces  and  set  up  again 
and  the  grinding  proceed  without  fear  of  disturbing  the 
measurement,  so  long  as  the  set  screw  in  the  point  finder  is 
not  moved.  It  is  often  necessary  during  grinding  to  loosen 
the  grinding  machine  from  the  lathe  bed,  slide  it  back  to 
adjust  something,  to  remove  disks  for  examination  of  the 
progress  of  the  work,  etc.  This  point  finder,  by  preserving 
the  distance  between  the  lathe  head  and  the  grinding 
machine,  enables  one  to  do  this  at  will,  and  again  find  his 
exact  point  of  measurement  simply  by  sliding  the  frame  of  the 
grinding  machine  forward  against  the  set  screw  of  the  point 
finder.  This  little  device  seems  absolutely  necessary  to  the 
highest  usefulness  of  the  machine. 

Lap  Wheels  and  Grinding  Stones. — I  began  my  work  of 
grinding  specimens  by  the  use  of  lap  wheels,  but  soon  dis- 
carded them  because  they  are  dirty.  They  cut  much  quicker 
than  stones,  however,  and  may  be  used  for  the  bulk  of  the 
work  when  much  grinding  of  very  hard  material  is  to  be 
done.    They  are  not  necessary  in  grinding  teeth,  bone,  etc.. 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS     461 

but  in  grinding  the  harder  fossils,  especially  those  impreg- 
nated with  the  silica,  and  in  some  geological  work  they 
become  necessary. 

The  best  lap  wheel  I  have  used  is  an  aluminum  wheel. 
Brass  or  iron  will  do  the  work,  but  aluminum  holds  the  grit 
better,  cuts  with  lighter  pressure,  and  does  the  work  quicker. 
In  using  these  I  have  fed  them  continuoush'  by  hand  with 
carborundum  powder  in  soapy  water,  using  a  brush. 

The  Stones. — Anyone  who  is  doing  much  grinding  should 
have  a  good  supply  of  stones.  I  have  a  pair  of  carborundum 
wheels,  a  pair  of  emery  wheels,  a  pair  of  India  oil  stones, 
and  a  pair  of  Arkansas  stones.  In  each  of  these  pairs  one 
is  fine  and  the  other  coarser  grit.  Every  stone  is  dressed 
to  a  perfect  face  on  the  lathe  head  where  it  is  to  do  its  work, 
with  a  black  diamond  held  in  the  slide  rest. 

These  stones,  when  put  in  good  shape,  seem  capable  of 
doing  an  unlimited  amount  of  work.  The  conditions  of  the 
grinding  prevents  them  from  getting  out  of  true.  All  that 
seems  necessary  is  to  roughen  them  a  bit  with  a  picking 
wheel  when  they  become  too  smooth  to  cut  well.  For  this 
purpose  a  much  smaller  picking  tool  than  the  smallest 
sold  for  the  general  mechanical  uses  seems  desirable.  This 
picking  wheel  has  sharp  teeth  of  the  hardest  steel  possible 
on  its  periphery.  It  is  held  in  a  handle  in  such  form  that 
the  wheel  is  free  to  turn.  In  use  it  is  held  against  the  rapidly 
rotating  stone  and  slowly  passed  over  its  entire  surface. 
It  may  be  held  in  the  hand  aided  by  a  tool  rest,  or  may  be 
arranged  for  use  in  the  slide  rest,  which  is  the  better  form 
for  this  work. 

Watering  the  Stones. — In  grinding,  the  stones  are  kept 
wet  in  running  ice  water.  A  balsam  that  is  too  soft  to  hold 
a  specimen  for  grinding  in  water  at  room  temperature  will 
hold  it  perfectly  in  ice  water,  because  it  is  much  harder  when 
cold.  For  this  purpose,  a  receptacle  for  ice  is  hung  on  the 
frame  that  holds  the  overhead  shaft,  and  filled  with  bits  of 
ice  and  then  filled  with  water.  Both  the  ice  and  the  water 
must  be  clean,  for  the  opening  in  the  tube  where  it  passes 
the   valve  which  regulates  the  flow  is  very  small,  and  a 


462  APPENDIX  CHAPTER  I 

little  bit  of  dirt  or  trash  might  stop  the  flow.  In  this  case 
the  specimen  being  gronnd  would  be  burned  instantly.  A 
bucket,  or  a  large  rubber  bag,  will  answer  for  this  purpose. 
Then  an  ordinary  rubber  tube  answers  to  conduct  the  water. 
It  is  best  to  have  this  rubber  tube  to  connect  with  a  metal 
tube  mounted  on  a  stand  that  may  be  placed  in  any  position 
wanted  to  deliver  the  water  to  the  stone.  This  metallic 
tube  is  provided  with  a  valve  for  the  regulation  of  the  flow. 
In  its  final  end  it  should  be  provided  with  a  brush  of  rather 
long  bristles,  into  which  the  w^ater  is  delivered  and  spread 
upon  the  stone.  This  brush  is  made  upon  a  short  tube 
fitted  into  the  end  of  the  metal  tube.  To  make  this  brush, 
first  cover  the  plain  part  of  the  small  brass  tube  with  thick 
shellac  dissolved  in  absolute  alcohol.  Place  a  layer  of  the 
bristles  around  it  and  wrap  them  tightly  wath  a  fine,  strong 
thread.  Then  place  more  shellac  over  this  and  another 
layer  of  bristles.  Continue  this  until  the  brush  is  large 
enough.  Then  wrap  thoroughly  with  a  cord  in  shellac, 
let  it  dry,  and  then  trim  it  up.  Two  of  these  have  served 
for  four  years  of  fairly  hard  usage. 

Waste  Water. — A  spatter  guard  is  made  by  bending  a 
|-inch  round  brass  tube  into  a  circle,  the  inner  diameter 
of  which  is  the  size  of  the  stones  used,  and  brazing  the  ends 
solidly  together.  Then  this  is  fixed  in  the  lathe  and  one- 
fourth  of  its  inner  circular  diameter  is  turned  away.  The 
grinding  stones  will  then  go  inside  this.  Then  this  piece  is 
provided  with  a  foot  and  hollow  post  and  fitted  to  the 
lathe  bed  with  a  washer  and  nut,  the  same  as  other  pieces 
are  attached.  This  catches  all  w^aste  water  and  through  a 
rubber  tube  attached  to  the  end  of  its  hollow  post  under  the 
lathe  bed  delivers  it  into  a  receptacle  so  placed  by  the  table 
as  to  receive  it.  This  prevents  all  of  the  spattering  of  water 
which  would  be  thrown  from  a  rapidly  revolving  wheel 
without  it.  If  it  should  be  inclined  to  run  over  when  a 
very  full  stream  is  wanted,  a  piece  of  rubber  dam  may  be 
stretched  over  the  foot  and  pulled  to  its  upper  end.  This 
may  be  caught  under  the  guard  in  fastening  it  to  the  lathe 
bed,  and  will  deliver  any  overflow  into  a  receptacle  placed 


THE  GRIXDIXG  OF  MICROSCOPIC  SPECIMENS     463 

to  receive  it.  In  this  way  nothing  is  wet  or  spattered  with 
water. 

Preparation  of  Material. — In  the  preparation  of  material, 
such  as  teeth,  bone,  etc.,  in  histological  work  of  ordinary 
delicacy,  the  specimen  is  first  ground  fiat  on  one  side  by 
hand  on  a  rough  stone  4  inches  in  diameter,  on  the  motor, 
and  finished  perfectly  flat  on  one  of  the  finer  stones  on  the 
lathe  head.  The  piece  is  then  washed  clean  and  placed  in 
absolute  alcohol  for  a  sufficient  time  to  remove  all  traces 
of  water,  or,  when  cracking  or  injury  from  shrinkage  is  not 
feared,  it  may  be  dried  in  the  warming  box.  Then  when 
dried  and  warmed  to  about  120°  F.,  it  is  ready  to  mount 
with  balsam  on  the  grinding  disk  for  grinding. 

Management  of  Balsam. — I  suppose  the  management  of 
balsam  will  always  be  a  difficult  problem  with  many  per- 
sons. ^Nlany,  hovrever,  learn  it  quickly.  One  may  take 
the  dry  balsam  and  dissolve  it  in  xylol,  and  filter  it  at  a 
high  temperature,  say  110°  or  120°  F.  Or  one  may  use  the 
prepared  balsam  for  microscopic  mountings.  In  either  case 
it  must  be  evaporated  until  stiff  enough  so  that  it  will  move 
rather  sluggishlv  at  110°  F.,  but  will  be  fluid  at  120°  or 
130°  F. 

Spiders  and  Dogs. — For  using  this  another  bit  of  apparatus 
is  necessary.  A  circular  piece  of  steel  made  flat  on  the  upper 
surface  is  mounted  on  three  legs  1|  to  2  inches  high.  The 
steel  disk  should  have  two  rows  of  holes  around  its  periphery, 
the  one  row  f  inch  inside  the  other.  A  hard  rolled  tool 
steel  wire,  or  rod  -fo  inch  in  diameter,  should  exactly  fit 
these  holes.  These  rods  should  now  be  bent  at  right  angles 
with  a  short  nib  on  the  end,  bent  again  at  right  angles,  so 
that  it  wdll  point  downward  when  the  free  end  of  the  rod 
is  set  into  one  of  the  holes.  The  length  between  these  two 
angles  should  vary  from  f  to  IJ  inches  in  three  dozen  or 
more  pieces  which  should  be  prepared.  The  end  which 
goes  in  the  holes  should  be  cut  so  that  it  will  not  quite 
reach  the  surface  of  the  table  when  dropped  into  the  holes 
with  the  end  of  the  nib  on  the  surface  of  the  circular  plate. 
These  rods  are  called  "dogs"  (Fig.  347.) 


464 


APPENDIX  CHAPTER  I 


With  this  arrangement  a  warming  box  arranged  with  a 
thermostat  to  maintain  an  even  temperature,  sufficiently 


Fig.  347 


The  spider  with  a  grinding  disk  upon  it  and  a  specimen  laid  on  and  secured  by  bent 
rods  called  dogs.  When  these  dogs  are  placed  and  pressed  down  through  the  holes  in 
the  disk  of  the  spider,  they  hold  fast.  With  a  little  pressure  of  the  finger  outward 
on  the  end  of  the  rod  below  the  disk  of  the  spider,  the  dog  slips  up  and  is  loose.  The 
disk  of  the  spider  is  three  inches  in  diameter. 


high  to  soften  the  stiff  balsam,  is  used.  The  specimen,  the 
balsam,  the  grinding  disk,  and  the  "spider"  are  placed  inside, 
and  allowed  to  rest  until  they  have  reached  the  temperature 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS     4G5 

desired.  Then  working  quickly,  a  sufficient  amount  of 
balsam  is  placed  on  the  grinding  disk,  and  the  specimen 
laid  on  it.  This  should  be  pressed  down  until  it  is  seen 
that  all  space  under  it  is  JSlled  with  balsam,  but  no  con- 
siderable excess  should  be  used.  It  is  well  if  this  rest 
so  in  the  warming  box  for  ten  or  fifteen  minutes  for  the 
balsam  to  soak  well  into  the  specimen.  Then  the  grinding 
disk,  with  the  specimens,  should  be  laid  on  the  spider  and 
one  of  the  dogs  dropped  into  one  of  the  holes  in  the  steel 
plate,  that  will  bring  its  nib  on  to  a  part  of  the  specimen 
chosen.  Then  another,  and  still  another,  should  be  placed, 
each  with  its  nib  on  a  different  part  of  the  specimen,  so 
that  every  part  of  it  may  be  pressed  flat  on  the  disk.  More 
dogs  should  be  added  if  necessary.  Now  each  in  turn  is 
pressed  down  a  little,  one  after  another,  until  all  are  exerting 
about  all  the  force  the  spring  of  the  rods  will  exert  without 
permanently  bending  them.  In  this  condition  the  whole 
thing  is  again  enclosed  in  the  warming  box. 

At  this  time  any  number  of  specimens  of  teeth  or  bits 
of  teeth,  bone,  etc.,  that  the  face  of  the  disk  will  hold  may 
be  placed  on  the  disk,  and  all  may  be  ground  together. 
Four  to  six  lengthwise  sections  of  incisor  or  cuspid  teeth 
may  be  placed  at  once,  or  eight  to  twelve  cross-sections. 
It  seems  to  be  best  practice,  however,  not  to  load  the  disk 
too  heavily.  Four  lengthwise  sections  will  grind  better 
than  six,  as  a  rule. 

Now,  after  the  loaded  disk  had  remained  in  the  warming 
box  until  all  balsam  that  will  come  has  been  squeezed  out 
from  under  the  specimens,  all  excess  of  balsam  should  be 
very  carefully  removed,  or  wiped  away,  close  up  against 
the  specimens.  Nothing  clogs  a  stone  and  stops  its  cutting 
more  effectually  than  balsam  smeared  over  it,  and  every 
excess  that  may  come  against  the  stone  should  be  got  out 
of  the  way. 

When  this  is  done  the  whole  thing  should  be  returned 

to  the  warming  box  for  from,  one  to  four  hours,  so  that  it 

may  dry  some  about  the  margins  at  least.    Then  it  may  be 

removed  from  the  warming  box  and  allowed  to  cool,  and 

30 


4G6  APPENDIX  CHAPTER  I 

await  convenience  in  grinding.  It  should,  however,  remain 
secured  on  the  si)i(ler  by  the  dogs  if  it  is  to  wait  more  than 
a  few  hours,  for  the  (Usi)()yition  of  dentine  to  warp  in  drying 
may  pull  some  i)art  of  the  specimen  from  the  disk.  Under 
these  conditions,  two  or  three  days,  or  a  week,  will  do  no 
harm. 

When  the  grinding  is  completed,  the  disk  is  removed  from 
the  machine  and  the  specimens  flushed  with  clean  water, 
and  dried  by  the  pressure  of  a  soft  napkin  folded  to  several 
thicknesses,  or  clean  pieces  of  waste  cotton  fabric  may  be 
used.  Then  the  disk  with  its  specimens  should  be  laid  in 
a  dish  and  sufficient  xylol  added  to  cover  it,  and  allowed 
to  rest  until  the  balsam  has  been  dissolved  and  the  specimens 
released.  This  will  usually  require  from  twenty  to  thirty 
minutes,  or  sometimes  as  much  as  an  hour.  When  the 
specimens  are  very  thin  they  loosen  much  quicker  than 
when  thick.  Any  material  not  penetrated  by  xylol,  as 
silicified  petrifactions  and  stones,  require  much  more  time. 

When  the  specimens  have  loosened,  they  are  ready  for 
permanent  mounting  for  microscopic  study. 

Rapidity  of  Grinding. — In  order  to  make  rapid  progress 
in  grinding  specimens,  one  should  have  six  to  ten  grinding 
disks,  nearly  as  many  spiders,  and  a  large  supply  of  dogs. 
The  machine  is  so  nearly  automatic  in  its  action  that  it 
needs  but  little  w-atching,  so  that  the  preparation  may  be 
going  on  while  the  grinding  is  in  progress.  One  of  the 
principal  points  that  needs  attention  is  the  flow  of  water. 
But  if  the  w^ater  and  ice  placed  in  the  receptacle  are  clean 
and  free  from  dirt  or  trash  that  may  stop  the  flow^  of  w^ater, 
the  only  care  is  that  the  quantity  of  water  is  kept  up.  The 
vessel  should  be  large  enough  to  hold  a  supply  for  several 
hours.  If  the  stone  should  run  dry,  the  specimen  would  be 
destroyed  in  a  few  seconds. 

Setting  the  Measurement  of  Grinding  Disks. — When  begin- 
ning any  considerable  series  of  grindings,  the  first  thing  of 
importance  is  to  try  out  and  obtain  a  record  of  the  measure- 
ments of  each  grinding  disk  for  the  particular  stone  that 
may  be  selected  for  finishing.     I  find  that  most  persons, 


THE  GRIXDTXG  OF  MICROSCOPIC  SPECIMENS     467 

after  some  practice,  prefer  to  use  a  fine  stone  for  the  entire 
grind.  In  grinding  teeth,  after  roughing  down  the  surface 
that  is  to  form  the  specimen,  the  back  is  also  ground  away 
to  a  flat  surface  that  will  better  accommodate  the  placing  of 
dogs  in  mounting  on  the  grinding  disks.  These  may  be 
made  quite  thin  and  reduce  the  grinding  with  the  fine  stone. 
Then  the  stone  selected  is  placed  in  the  lathe  head,  seeing 
to.it  carefully  that  the  face  of  the  stone  is  clean.  Then  the 
grinding  machine  is  brought  up  in  contact  with  the  set  screw 
of  the  point  finder.  The  tailpiece  is  placed  in  position  and 
pushed  up  so  as  to  make  some  pressure  on  the  shaft.  Then, 
with  the  large  nut  the  shaft  is  so  adjusted  that  the  grinding 
disk  being  tried  comes  close  to  the  stone  but  does  not  touch 
it.  Now  start  the  machine  and  note  the  running  carefully, 
and  while  doing  so  catch  the  adjusting  nut  of  the  micrometer 
and  move  it  one-thousandth  at  a  time,  and  listen  for  the 
first  touch  of  the  disk  to  the  stone.  The  moment  this  is 
heard,  quickly  reverse  the  movement  of  the  adjusting  nut, 
and  separate  the  disk  from  the  stone.  Try  this  again  and 
again,  until  you  feel  very  certain  of  having  detected  the 
first  touch  of  the  stone  on  the  disk  by  moving  the  adjusting 
nut  half  or  a  quarter  of  toV"o  inch.  At  last,  while  it  is  touch- 
ing, stop  the  machine  in  a  position  to  see  the  finger  on  the 
adjusting  nut,  and  read  the  measurement  and  enter  it  on 
your  record  for  that  disk.  In  setting  for  a  grind  with  this 
disk,  turn  the  adjusting  nut  so  as  to  draw  the  grinding  disk 
back  from  the  stone  1-0^7 o"  inch.  When  the  specimens  to 
be  ground  are  mounted  on  this  disk,  place  it  back  on  the 
machine,  start  it,  seeing  that  the  iced  water  is  running  first, 
and  let  it  run  until  it  ceases  to  cut,  which  it  will  do  when 
the  forward  movement  of  the  shaft  is  stopped  by  the  con- 
tact of  the  adjusting  nut  of  the  micrometer  with  the  rear 
bearing  of  the  shaft. 

Then  remove  the  disk  and  examine  the  specimens  care- 
fully. If  the  placement  has  been  accurate,  the  specimens 
will  be  too  thick.  Replace  the  disk  carefully  and  turn  the 
nut  forward  so  as  to  grind  one-thousandth  of  an  inch  thinner, 
or  one  mav  do  onlv  a  half  of  one-thousandth  at  a  time. 


468  APPENDIX  CHAPTER  I 

Repeat  this  until  the  section  seems  to  be  thin  enough. 
Then  remove  and  mount  the  sections  and  judge  them  with 
the  microscope.  By  this  time  one  will  have  arrived  at  an 
accurate  measurement  of  this  disk,  and  the  record  will  be 
trustworthy  for  other  grinds,  and  will  not  have  to  be  repeated 
until  the  wearing  of  the  stone  begins  to  leave  the  specimens 
a  bit  thick.  Then  a  half-thousandth  of  an  inch  will  bring 
it  right.  And  so  on,  and  on.  Each  disk  will  be  treated  in 
the  same  way  for  each  stone  used,  and  if  one  is  doing  much 
grinding  all  will  be  running  on  their  records,  and  all  go 
smoothly.  Recently  a  man  who  was  grinding  sections  of 
teeth  for  me  made  all  of  the  preparations,  preparatory 
grindings,  and  disk  mounts,  ground  and  removed  from  the 
disks  ready  for  mounting  forty  full-length  sections  of  central 
incisors  in  six  hours,  and  had  his  lunch  during  the  time. 
Every  section  was  complete,  was  even  in  thickness  in  every 
part,  and  all  practically  the  same  thickness — a  thickness 
chosen  for  the  special  studies  in  hand. 

Grinding  Frail  Material. — While  the  machine  facilitates 
the  production  of  the  more  ordinary  sections  to  such  a 
degree  as  to  be  indispensable  to  one  having  many  grindings 
to  do,  it  is  in  the  production  of  sections  of  very  frail  material 
that  the  grinding  machine  stands  out  as  vastly  superior 
to  other  methods  of  grinding.  In  the  study  of  caries  of 
enamel  in  which  disintegration  has  rendered  the  remaining 
tissue  very  frail  and  likely  to  fall  to  pieces  before  it  is  suffi- 
ciently thin,  we  may  obtain  the  required  thinness  and  yet 
retain  all  of  the  tissue.  I  have  also  produced  exceedingly 
fine  sections  of  salivary  calculus,  and  equally  good  sections 
from  small  crumbs  of  serumal  calculus.  The  production 
of  these  is  slow,  but  fairly  certain  of  good  results. 

Also  in  grinding  sections  of  fossil  teeth,  fossil  woods,  and 
the  like,  in  which  very  fine  sections  are  too  brittle  to  be 
handled  in  any  way  except  as  stuck  to  glass,  the  machine 
gives  excellent  results.  In  geological  work  it  practically 
removes  the  difficulties.  Good  sections  of  the  very  brittle 
stones  can  be  made  with  fair  safety  by  grinding  on  the  cover- 
glass. 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS     469 

Plans  for  Grinding  Frail  Material. — Much  very  desirable 
material  for  microscopic  investigation  will  be  found  that  is 
so  frail,  or  at  least  so  brittle,  when  reduced  to  sections  thin 
euough  for  microscopic  investigation,  that  it  will  crumble 
to  pieces,  either  in  the  grinding  or  in  the  mounting,  by  the 
ordinary  processes.  For  grinding  and  mounting  such 
material  the  following  processes  have  been  slowly  evolved. 
These  may  be  divided  into  the  balsam  process  and  the 
shellac  process.  Such  material  that,  when  made  fast  to 
a  cover-glass  and  ground  in  hard  balsam,  is  not  liable  to 
go  to  pieces  when  this  hard  balsam  is  softened  by  sticking 
the  specimen  and  glass  cover  to  a  glass  slide  may  be  ground 
in  hard  balsam.  If,  howveer,  the  different  parts  are  liable 
to  separate  and  change  position  when  the  balsam  softens, 
shellac  should  be  used  for  the  grinding.  I  have  had  some 
very  sorrowful  failures  in  grinding  rare  specimens  of  enamel 
that  had  no  cementing  substance  between  the  enamel 
rods  in  hardened  balsam.  For  when  the  softer  balsam  was 
added  to  mount  the  specimen  on  the  glass  slide,  the  hard 
balsam  was  softened  and  the  enamel  rods  floated  out  of 
position.  All  such  material  as  will  not  hold  together  strongly 
enough  to  prevent  this  should  be  ground  in  shellac. 

To  grind  in  hard  balsam,  the  one  side  of  the  specimen  may 
be  ground  flat  on  the  rough  stone  and  then  dried  out  in 
absolute  alcohol.  Then  the  ground  side  should  be  saturated 
to  sufficient  depth  with  soft  balsam,  and  laid  aside  until 
the  balsam  has  become  hard  enough  to  grind  smoothl}^ 
Then  the  grinding  and  polishing  of  this  first  side  should  be 
completed  by  grinding  away  all  balsam  from  the  immediate 
surface,  and  sufficiently  into  the  substance  of  the  specimen 
to  produce  a  clean,  smooth  surface  of  the  material.  When 
this  has  been  done,  and  the  surface  dried,  it  should  be 
mounted  on  an  ordinary  cover-glass,  the  thickness  of  which 
should  have  been  measured  and  recorded.  In  this  mounting 
the  cover-glass  should  be  laid  on  a  spider  and  weight  enough 
placed  upon  it  to  insure  a  perfect  fit  of  the  surface  of  the 
glass.  This  should  be  subjected  to  about  120°  F.  heat  for 
from  one  to  five  or  six  hours,  for  the  purpose  of  expressing 


470  APPENDIX  CHAPTER  I 

the  last  bit  of  balsam  possible  from  between  the  specimen 
and  the  cover-glass.  Then  it  may  rest,  awaiting  the  con- 
venience of  the  operator,  for  several  days,  but  the  balsam 
must  not  be  allowed  to  become  "brittle  hard,"  because  in 
that  case  it  loses  toughness.  All  excess  of  balsam  about 
the  margins  of  the  specimen  should  be  carefully  removed 
to  facilitate  the  hardening  of  that  which  remains,  and  espe- 
cially so  that  it  may  not  come  in  contact  with  the  grinding 
stone,  stick  to  its  surface,  and  interfere  with  the  cutting. 

Good  judgment  must  be  acquired  by  practice  as  to  the 
hardening  of  balsam  and  shellac  in  these  grinding  processes. 
The  best  idea  of  it  that  can  be  given  in  words  is  this.  The 
balsam  or  the  shellac  imist  have  become  firm  enough  so  that 
it  will  not  drag  or  allow  the  specimen  to  move  while  grinding 
in  iced  water.  Neither  must  it  become  hard  enough  to  becoine 
brittle,  for  then  it  becomes  liable  to  break. 

When  ready,  the  specimen  is  mounted  on  the  grinding 
disk.  This  is  done  by  first  cleansing  the  disk,  finishing  with 
x;y'lol,  and  then  sealing  the  cover-glass  to  this  with  soft 
balsam.  This  should  be  placed  on  the  spider  and  well 
weighted  down  with  dogs.  All  excess  of  balsam  should  be 
carefully  wiped  away  from  the  margins  of  the  cover-glass. 
This  may  be  quickly  dried  at  120°  F.,  or  more  slowly  at 
room  temperature.  It  should,  however,  be  warmed  for 
a  half  hour  or  more,  for  the  purpose  of  expressing  as  much 
balsam  as  possible.  This  cover-glass  will  be  well  held  for 
grinding  in  iced  water  with  only  a  little  drying  about  the 
margins,  if  all  excess  of  balsam  is  cleaned  away  closely. 
The  balsam  should  not  become  very  hard. 

If  the  specimen  is  of  considerable  bulk  and  of  a  quality 
of  material  that  can  be  cut  with  a  steel  saw,  the  disk  may 
be  caught  in  a  vice  "with  leather-cushioned  jaws  to  avoid 
bruising,"  and  the  bulk  of  the  material  removed  with  a 
jeweller's  saw,  leaving  only  a  moderately  thin  section  for 
grinding.  Or  if  the  material  is  very  hard,  as  stones,  silicified 
fossils,  etc.,  the  disks  may  be  mounted  upon  the  slide  rest 
and  cut  with  the  slicing  disks,  to  be  described  later. 

The  specimen  is  now  ready  for  the  final  grinding.     The 


THE  GRIXDIXG  OF  MICROSCOPIC  SPECIMEXS     471 

record  for  measurement  with  the  particular  stone  to  be 
used  in  finishing  has  been  made,  tried  out  on  unimportant 
material,  and  the  cover-glass  has  been  measured  and  its 
record  made.  With  this  data,  the  disk  is  screwed  to  its 
place,  the  micrometer  turned  to  the  proper  measurement 
for  the  finish,  the  iced  water  arranged,  the  machine  set  in 
motion,  and  it  will  do  the  rest.  When  coarser  stones  are 
used  for  cutting  away  considerable  material,  I  find  those 
with  just  a  little  experience  prefer  to  gauge  the  amount 
of  the  cutting  by  the  eye  for  the  coarse  stone. 

Removal  of  the  Cover-glass  from  the  Disk. — I  remove  the 
cover-glass  with  the  specimen  from  the  grinding  disk  in 
two  different  ways,  as  seems  at  the  time  best. 

First,  the  grinding  disk  is  placed  on  a  heated  piece  of 
metal  that  will  warm  the  grinding  disk  quickly.  Have  a 
stick  of  rather  soft  wood  ready,  the  end  of  which  is  cut  to 
a  rather  sharp  angle  and  thinned  down  almost  in  the  form 
of  a  blade.  When  the  grinding  disk  begins  to  warm,  catch 
the  margin  of  the  cover-glass  with  the  end  of  the  stick  and 
begin  to  make  steady  pressure.  As  the  disk  warms,  so  as  to 
soften  the  balsam,  the  cover-glass  will  begin  to  move  under 
the  steady  pressure,  slowly  at  first,  but  more  rapidly  later, 
and  will  slide  oft'  the  grinding  disk  before  the  specimen 
is  loosened.  For  this  plan  the  cover-glass  should  be  pretty 
strong,  one  and  one-half  to  two  thousandths  of  an  inch 
thick.  Otherwise  there  will  be  great  danger  of  breaking 
it.  It  is  well  in  some  cases  to  run  just  a  little  xylol  around 
the  margins  of  the  cover-glass  and  partially  dissolve  the 
balsam  that  has  become  driest  before  the  heating.  Great 
care  must  be  taken  not  to  allow  the  xylol  to  spread  on  to  the 
specimen,  for  it  would  loosen  it  very  quickly. 

The  specimen  is  then  turned  downward  and  placed  on  a 
tiny  drop  of  balsam  on  a  glass  slide,  and  quickly  pressed 
down  close  and  level.  As  the  new  balsam  will  soften  the 
old,  it  should  not  be  moved  further  than  quickly  to  apply 
a  light  spring  clip  to  hold  it  steady.  The  parts  of  the  speci- 
men are  less  likely  to  move  if  this  is  laid  on  ice  for  an  hour 
or  more. 


472  APPENDIX  CHAPTER  I 

The  Use  of  Shellac. — In  the  second  plan  shellac  is  used 
instead  of  balsam  for  hardening  the  specimen  and  holding 
its  parts  together  in  the  first  grinding.  This  part  of  the 
work  is  other\vise  done  in  the  same  way.  The  drying  of  the 
shellac  requires  more  time  usually  than  the  balsam. 

The  attachment  of  the  cover-glass  to  the  grinding  disk 
is  done  in  the  same  way  as  when  balsam  is  used  to  hold  the 
specimen  on  the  cover-glass — that  is,  with  balsam.  The 
grinding  proceeds  similarly  in  every  respect. 

In  the  removal  of  the  cover-glass  from  the  grinding  disk, 
and  mounting  the  specimen,  comes  the  important  differences 
in  the  two  processes.  Xylol  dissolves  balsam  very  quickly. 
But  xylol  does  not  dissolve  shellac  at  all.  Therefore,  in- 
stead of  pushing  the  cover-glass  of  the  grinding  disk,  the 
disk  is  laid  in  xylol  and  the  balsam  dissolved  out.  In  this 
there  is  no  danger  of  detaching  or  moving  the  specimen  if 
the  handling  is  careful.  When  cleaned,  it  is  inverted  upon 
a  glass  slide  on  a  drop  of  balsam  without  fear  of  movement 
of  parts  of  the  specimen,  no  matter  how  frail. 

The  Preparation  of  Shellac. — To  keep  shellac  in  condition 
for  this  work  has  some  difficulties.  The  dry  scales  should 
be  dissolved  in  absolute  alcohol  so  as  to  make  a  moderately 
thick  varnish.  It  should  then  be  filtered  at  a  temperature 
of  110°  to  120°  F.,  or  be  made  thinner  and  filtered  at 
room  temperature.  Great  care  should  be  exercised  to  keep 
the  filtrate  from  exposure  to  a  damp  atmosphere,  for  it 
absorbs  water  readily  and  then  will  throw  down  fine  crys- 
tals, which  destroy  its  value  for  microscopic  purposes. 

After  being  filtered  it  should  be  evaporated  in  a  close 
warming  box  at  about  110°  to  120°  F.,  to  the  consistence  of 
syrup.  In  doing  this  it  is  well  to  divide  the  supply  into  two 
or  three  grades — a  thinner,  medium,  and  a  thicker  solution. 
The  thinner  solution  will  be  used  for  saturating  frail  speci- 
mens before  any  cutting  is  done.  The  thicker  solutions  for 
attaching  specimens  to  the  cover-glass  for  grinding.  The 
medium  solution  for  either  purpose,  as  the  material  may 
seem  to  require. 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS     473 

The  Grinding  from  Crumbled  Material. — There  is  often 
important  material  for  investigation  that  can  be  had  only 
in  very  small  crumbs,  or  broken  pieces,  such  as  serumal  cal- 
culus, sands,  crumbled  bits  of  strange  stones,  or  mixtures  of 
such  material  as  is  found  in  some  of  the  coarser  sands.  These, 
on  microscopic  investigation,  ma^'  tell  important  stories  as 
to  their  origin  and  throw  important  light  upon  geological 
questions.  In  addition  to  the  ordinary  microscopic  observa- 
tion, the  polariscope  may  be  turned  on  these,  and  reveal 
important  facts  as  to  their  origin  and  structure.  Also  many 
things  will  be  found  in  botanical  work,  such  as  obtaining 
sections  of  small  seeds,  and  the  like,  which  will  give  important 
information. 

Having  done  a  few  of  these  grindings,  especially  of  the 
very  frail  dental  material,  such  as  serumal  calculus,  extremely 
frail  fossil  teeth,  etc.,  plans  of  work  more  or  less  well  adapted 
have  been  developed. 

For  instance,  I  have  obtained  excellent  sections  of  serumal 
calculus,  which  can  be  had  only  in  small  crumbs  or  flakes, 
in  this  wise.  A  small  collection  of  these  bits  are  first  im- 
mersed for  a  time  in  absolute  alcohol,  or  until  all  air  has 
been  removed  if  they  are  dry,  or  if  the}'  are  freshly  gathered, 
until  all  water  has  been  removed.  Then  a  cover-glass  is 
prepared  by  covering  its  central  part  with  the  thicker 
solution  of  shellac,  and  these  crumbs  are  placed  in  this, 
in  what  seems  to  be  the  best  position  for  obtaining  sections. 
These  are  allowed  to  soak  full  of  the  shellac,  under  a  close 
cover,  and  then  uncovered  to  dry  up.  Then,  if  some  of  the 
pieces  seem  to  need  it,  more  shellac  is  added  from  time  to 
time,  until  the  embedding  seems  sufficient.  This  may  be 
dried  at  room  temperature,  or  in  the  warming  oven  at  110° 
to  120°  F.  Shellac  should  not  be  subjected  to  much  higher 
temperatures  for  a  considerable  time,  because  continued 
high  temperature  for  many  days  together  seems  to  injure 
the  strength. 

When  this  is  sufficiently  hard  for  smooth  grinding,  and 
before  it  has  become  too  brittle  (determining  this  point  re- 
quires some  experience),  the  preparation  is  cemented  to  the 


474  ArrENDiX  C  If  AFTER  I 

grinding  disk  with  halsam  and  ground  to  such  a  point  as 
seems  most  favorable  for  obtaining  sections.  This  point  is 
to  be  determined  by  frequent  removal  of  the  disk  from  the 
machine  and  examination  of  the  exposed  surfaces  of  the 
several  pieces. 

When  this  })art  is  done,  the  cover-glass  is  dissolved  off 
of  the  grinding  disk  by  xylol.  Then  another  cover-glass 
is  attached  to  the  surface  tvith  the  least  possible  amount  of 
shellac.  This  in  turn  is  dried  to  the  right  consistence.  Then 
the  last  cover-glass  placed — that  is,  the  one  on  the  side  that 
has  been  ground — is  secured  to  the  grinding  disk  with  balsam. 
When  this  has  set  it  is  placed  on  the  machine  and  the  first 
cover-glass  is  ground  away  and  the  section  ground  to  the 
required  thinness.  They  are  again  dissolved  off  of  the  grind- 
ing disk,  and  may  be  at  once  mounted  in  balsam  on  the 
microscopic  slide. 

Difficulties  in  Grinding. — In  the  grinding  of  material 
enveloped  in  shellac,  or  in  balsam,  either  of  these  materials 
are  apt  to  gum  up  the  stone  and  stop  the  cutting,  or  render 
the  grinding  very  slow.  When  this  is  from  balsam,  it  may 
be  quickly  removed  after  drying  the  stone  by  washing  w4th 
xylol  on  a  brush,  or  a  bit  of  cloth,  while  the  stone  is  slowly 
revolved. 

When  clogged  with  shellac,  the  washing  is  done  with 
absolute  alcohol.  This  requires  much  more  time,  and  some 
advantage  may  be  obtained  b}^  using  pumice  stone  wuth  the 
cloth  or  with  cork.  After  rubbing  with  pumice  stone,  a 
very  thorough  washing  with  alcohol  should  be  made  to 
remove  the  last  particles  of  pumice,  before  re-beginning  the 
grinding.  Even  with  this,  the  ground  surface  is  apt  to  be 
rough  or  scratched  for  a  time  by  particles  of  the  pumice 
lodged  on  the  stone.  These  will  soon  disappear,  how'ever. 
Yet  the  pumice  should  not  be  used  in  the  last  portion  of 
the  grinding. 

With  much  grinding  of  hard  substances,  the  surfaces  of 
the  stones  become  worn  so  smooth  that  the}'  do  not  cut 
well.  Then  the  picking  tool  should  be  run  over  the  surface 
until  it  is  perceptibly  roughened.    This  will  cause  the  stone 


THE  GRIXDIXG  OF  MICROSCOPIC  SPECIMENS     475 

to  cut  briskly  for  a  considerable  time,  and  at  first — following 
such  sharpening — the  ground  surface  of  the  specimen  is 
likely  to  be  full  of  scratches.  In  that  case  a  smooth  stone 
should  be  used  for  the  finishing. 

Much  care  should  be  taken  in  keeping  the  stones  in  good 
condition.  Except  in  the  ways  mentioned,  no  dirt  or  grit 
should  be  allowed  to  come  in  contact  with  their  surfaces. 
A  single  particle  of  grit  lodged  in  the  surface  of  the  stone 
will  fill  the  whole  surface  of  the  ground  section  with  scratches. 
Although  I  shut  up  my  stones  in  a  close  fitting  drawer,  I 
find  it  necessary  to  cover  each  with  a  close  fitting  cloth  that 
is  so  closely  woven  as  to  exclude  all  dust. 

In  taking  care  of  the  machine  itself,  one  cannot  be  too 
careful.  All  of  the  bearings  of  the  lathe  head  and  of  the 
grinding  machine  should  be  swaddled  with  candle  wick 
saturated  with  oil  to  prevent  the  ingress  of  gritty  particles. 
This  is  especially  needful  when  using  the  aluminum  saws 
and  feeding  them  with  carborundum  powder.  Then  every 
bearing  about  the  whole  machine  should  be  especially  pro- 
tected to  prevent  the  possibility  of  getting  grit  in  the  bear- 
ings. Carelessness  in  such  a  matter  will  quickly  ruin  a 
fine  bit  of  mechanism.  But  with  this  care,  such  a  machine 
should  continue  to  do  its  work  well  for  a  lifetime  (Figs.  348 
and  349). 

The  Slicing  Mechanism. — This  is  an  arrangement  for  slic- 
ing very  hard  substances  which  cannot  be  cut  with  the 
ordinary  steel  saw — such  as  the  enamel  of  teeth,  silicified 
fossils,  rocks,  etc.  This  consists  of  an  aluminum  disk 
fitted  to  the  lathe  head,  and  surrounded  by  a  special  form 
of  spatter  guard  that  admits  of  the  use  of  the  periphery 
for  cutting,  and  an  object  holder  fixed  upon  the  slide  rest 
of  the  lathe.  The  object  holder  consists  of  a  clamp  that 
grasps  a  brass  tube  slotted  at  the  free  end  in  which  teeth, 
or  other  objects  may  be  made  fast  with  plaster  of  Paris  or 
sealing  wax  for  slicing.  Or  in  place  of  this  a  brass  man- 
dril, upon  the  end  of  which  there  is  a  threaded  nipple 
by  whidi  any  of  the  grinding  disks  may  be  attached. 
These  are  fixed  in  the  position  of  the  ordinary  tool  post, 


476 


APPENDIX  CHAPTER  I 


and  may  be  swung  horizontally  to  any  possible  position 
in  relation  to  the  aluminum  disk.  An  object  can  there- 
fore be  so  placed  on  the  disk  as  to  be  cut  in  any  direction 
desired.  Usually  these  are  fixed  upon  the  disk  with  sealing 
wax.  In  using  the  aluminum  disk  it  is  fed  with  carborundum 
powder  suspended  in  soapy  water  to  give  it  some  stickiness. 

Fia.  348 


Figs.  348  and  3 19. — Arrangement  for  slicing  very  hard  material.  Fig.  348  is  the  more 
ordinary  view  of  the  machine  with  the  slide  rest  and  object  holder  in  position.  In 
Fig.  349  the  lathe  is  turned  about  to  give  a  better  view  of  the  slide  rest,  object  holder, 
spatter  guard,  and  aluminum  disk.  In  these  illustrations  the  slotted  tube  is  used 
(see  text)  to  hold  the  object  being  cut.  Notice  that  the  disk  used  for  cutting  is  sur- 
rounded by  a  spatter  guard  which  is  open  for  a  space  at  one  side  so  that  the  periphery 
of  the  disk  may  be  used  in  cutting.  This  guard  gathers  all  water  and  grit  used  in 
cutting,  and  delivers  it  into  the  pan  below  through  its  hollow  post.  When  doing 
this  kind  of  work  all  of  the  bearings  of  the  machine  should  be  carefuUy  wrapped 
(swaddled)  to  keep  them  safe  from  intrusion  of  grit. 


This  is  applied  with  a  brush  by  hand,  and  is  kept  going  so 
constantly  as  to  prevent  the  disk  from  running  dry.  The 
ordinary  aluminum  plate,  of  twenty-four  to  thirty  gauge, 
may  be  used  for  making  these.    They  are  first  cut  in  circles 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS     477 

by  hand,  as  large  as  the  lathe  will  swing  (4  inches),  and  then 
are  cut  down  to  3 J  inches  with  a  tool  in  the  slide  rest.  These 
are  quickly  made  when  wanted.  They  wear  out  rapidly, 
and  yet  one  of  them  will  do  much  cutting  of  very  hard 
substances,  and  do  it  accurately  and  delicately.  Rings  may 
readilv  be  cut  from  the  ordinary  test-tubes  without  special 


Fig.  349 

i 
I 

■eg|L\ 

\f)h^ 

^an  -^\-.^^ 

<^^^^S^  \^Hr 

LJ* 

■^>'> 

ml- 

^_aB»i 

■^^j0^^ 

T 

in. 

000^'n^2 

1 

' 

^ 

danger  of  breaking.  The  crown  of  a  molar  tooth  may  be  cut 
into  many  slices;  fossil  teeth,  silicified  fossil  woods,  stones, 
etc.,  may  readily  be  sliced  as  thin  as  they  can  be  handled 
in  the  after-work  of  preparation. 


APPENDIX  CHAPTER  II 


THE  THEORY  OF  HISTOLOGICAL  TECHNIQUE 

The  first  requirement  of  histological  technique  is  to 
obtain  a  general  view  of  the  theory  of  procedure.  Many 
beginners  make  the  mistake  of  supposing  that  directions 
for  histological  technique  can  be  followed  like  the  receipts 
of  a  cook  book,  or  the  directions  for  an  experiment  in  chem- 
istry. This  is  very  seldom  the  case,  and  while  it  is  always 
necessary  to  follow  directions  accurately,  it  is  still  more 
necessary  to  follow  them  intelligently.  All  histological 
methods  require  judgment.  For  instance  the  length  of 
time  required  for  xylol  to  replace  absolute  alcohol  in  a  block 
of  tissue  which  is  to  be  embedded  depends  upon  the  size 
of  the  piece,  the  character  of  the  tissue,  the  temperature, 
and  possibly  some  other  factors.  It  is  therefore  impossible 
to  say  exactly  what  time  would  be  required,  and  the  experi- 
menter must  use  the  judgment  which  has  been  acquired 
as  the  result  of  experiment.  In  the  same  way  no  experi- 
menter can  make  up  a  stain  and  be  sure  that  it  will  work 
exactly  like  the  last  lot  made  by  the  same  formula  until 
he  has  tried  it.  Even  with  the  same  stain  the  length  of 
time  required  for  staining  a  section  depends  upon  the  thick- 
ness of  the  section,  the  character  of  the  tissue,  and  the  pre- 
liminary technique  it  has  been  through.  So  that  all  time 
directions  must  be  considered  as  approximate,  and  to  be 
successful  the  experimenter  must  study,  first,  the  object 
to  be  obtained  by  the  use  of  each  reagent,  and  the  peculiar 
action  of  the  reagent  upon  the  tissue. 

For  observation  with  the  compound  microscope  trans- 


THE  THEORY  OF  HISTOLOGICAL  TECHNIQUE       479 

mitted  light  is  ordinarily  used.  The  object  must  therefore 
be  thin  and  transparent  enough  to  allow  the  light  to  pass 
through  it.  The  higher  the  magnification  the  smaller  the 
field,  that  is,  the  smaller  the  portion  of  the  tissue  that  can 
be  seen  at  one  time,  and  the  less  depth  of  focus,  and  con- 
sequently the  thinner  the  sections  must  be.  A  section  that 
would  be  excellent  for  study  with  the  f  objective  may  be 
almost  valueless  under  a  yV>  ^nd  sections  that  are  splendid 
under  the  yV  might  be  of  little  value  under  the  f .  In  other 
words,  the  thickness  of  the  section  should  be  related  to  the 
magnification  with  which  it  is  to  be  studied,  and  to  the  size 
of  the  structural  elements  which  make  up  the  tissue.  For 
the  study  of  the  organs  and  tissues  of  multicellular  organisms 
there  are  three  general  methods — (1)  teasing,  (2)  macera- 
ting, and  (3)  sectioning. 

Teasing. — In  this  method  a  small  portion  of  the  living 
tissue  is  torn  apart  with  two  needles  in  a  drop  of  normal 
salt  solution  or  some  indifferent  medium  which  will  not 
aifect  the  tissue.  In  this  way  it  is  spread  into  a  thin  film 
and  squeezed  a  little  between  a  slide  and  cover-glass  so 
as  to  separate  the  structural  elements  when  they  may  be 
directly  observed.  Of  course,  in  stud^'ing  such  a  preparation 
it  must  be  remembered  that  the  tissue  has  been  forcibly 
torn  apart  and  effects  of  violence  must  be  looked  for.  These 
often  bring  out  facts  of  structure  which  would  not  otherwise 
be  as  easily  seen.  After  teasing  the  living  tissue,  staining 
agents  may  be  used  to  facilitate  the  study  of  structure. 
The  fresh  tissues  are  often  so  transparent  and  made  up  of 
substances  of  so  near  the  same  refracting  index  that  very 
little  structure  can  be  made  out  without  the  use  of  staining 
agents.  It  must  be  borne  in  mind  that  staining  agents 
are  of  two  classes,  diffuse  and  selective.  A  diffuse  stain 
gives  an  even  color  to  all  of  the  tissue  and  facilitates  the 
study  chiefly  by  rendering  it  less  transparent.  A  selective 
stain  combines  more  readily  with  one  portion  of  the  tissue 
than  another,  rendering  it  more  conspicuous.  Selective 
stains  therefore  must  be  thought  of  as  chemical  agents  which 
combine  with  parts  of  the  cell  or  tissue  and  demonstrate 


480  APPENDIX  CHAPTER  II 

chemical  differences  in  the  structural  elements.  For  in- 
stance, basic  anilines  react  with  the  chromatin  of  the  nucleus, 
producing  a  colored  compound.  The  stain  may  then  be 
washed  out  of  the  section,  leaving  only  the  nuclei  colored. 
Acid  anilines  in  general  are  diffusive  stains  giving  a  general 
color  to  the  cytoplasm.  In  a  similar  way  certain  stains 
will  react  only  or  chiefly  with  intercellular  substances, 
rendering  them  more  conspicuous.  For  staining  freshly 
teased  specimens  methyl  green,  the  formula  for  which  will 
be  found  under  the  paragraph  on  stains,  is  an  excellent 
agent.  Teased  specimens  are  never  very  permanent,  though 
they  may  be  preserved  for  a  considerable  length  of  time 
by  mounting  in  glycerin  or  glycerin  jelly  and  putting  a  ring 
of  varnish  or  white  lead  around  the  edge  of  the  cover-glass 
so  as  to  prevent  evaporation. 

Maceration. — When  an  organ  is  composed  of  more  than 
one  tissue  the  structural  elements  may  be  separated  by 
selecting  an  agent  which  will  act  upon  one  and  not  upon 
the  others;  for  instance,  the  muscle  fibers  of  a  voluntary 
muscle  may  be  separated  by  treating  a  piece  of  tissue  with 
dilute  alkali,  which  w^ill  soften  and  dissolve  the  connective 
tissue,  allowing  the  muscle  fibers  to  separate.  In  a  similar 
way  dilute  alcohol  will  soften  the  cementing  substance  be- 
tween the  epithelial  cells.  By  first  treating  a  piece  of  tissue 
with  the  proper  agent  and  then  teasing,  the  form  of  the 
structural  elements  of  the  tissue  can  be  made  out.  By 
treating  a  portion  of  connective  tissue  containing  both 
white  and  elastic  fibers  with  dilute  hydrochloric  or  acetic 
acid,  which  dissolves  the  white  fibers,  elastic  fibers  which 
could  otherwise  not  be  seen  may  be  made  out.  Macer- 
ating and  teasing  methods  are  of  great  assistance  to  the 
study  of  tissues  in  sections,  and  it  would  be  often  very 
difficult  to  obtain  true  ideas  of  structure  from  sections 
without  their  assistance. 

Sectioning. — For  the  study  of  the  structural  elements  in 
their  relation  to  each  other  in  the  tissue  sectioning  is  the  one 
method.  As  they  exist  in  the  body,  however,  some  of  the 
tissues  are  too  soft  and  others  too  hard  to  allow  the  cutting 


THE  THEORY  OF  HISTOLOGICAL   TECH XI QUE     481 

of  a  thin  enough  shoe  without  disturbing  the  relation  of  the 
structural  elements.  They  must  therefore  be  put  through 
rather  an  elaborate  process  in  which  the  object  of  every 
step  must  be  understood. 

Dissecting. — First  of  all,  the  material  for  histological  work 
must  be  absolutely  fresh,  that  is,  living.  It  must  be  remem- 
bered that  living  cytoplasm  is  chemically  different  from 
dead  cytoplasm,  and  as  soon  as  death  occurs  postmortem 
changes  begin  which  gradually  destroy  the  structure.  The 
period  from  death  to  the  beginning  of  histological  methods  of 
preparation  should  be  measured  in  minutes,  not  in  hours. 
Tissues  that  have  been  dead  for  a  few  hours  will  not  react 
with  the  staining  agents  so  as  to  produce  the  brilliant 
specimens  that  can  be  obtained  from  fresh  material,  and 
often  a  few  days  will  render  material  entirely  useless  except 
for  the  grosser  anatomical  relations.  The  specimens  to  be 
studied  should  be  dissected  while  the  cells  of  the  tissue 
are  still  alive,  and  in  doing  so  the  greatest  care  should  be 
used  not  to  disturb  the  relation  of  the  tissues. 

Fixing. — Histologically  this  word  means  killing.  After 
dissecting  out  the  tissue  to  be  studied,  and  while  the  cells  are 
still  alive,  it  must  be  immersed  in  some  liquid  that  will 
kill  the  cells  and  fix  their  structure  as  w^hen  alive.  The 
pieces  should  be  made  small  enough  for  the  fixing  agent 
to  penetrate  them  rapidly,  and  the  size  of  the  piece  that  can 
be  used  depends  upon  the  density  of  the  tissue,  its  character, 
and  the  nature  of  the  reagent.  Some  fixing  agents  are  very 
much  more  penetrating  than  others.  All  fixing  agents 
coagulate  or  set  the  cytoplasm  and  tend  to  prevent  shrink- 
age. The  success  of  all  the  following  steps  and  the  value 
of  the  specimen  for  the  study  of  detail  of  structure  depend 
upon  the  perfection  of  fixation. 

The  fixing  agents  most  commonly  used  are  bichloride  of 
mercury,  potassium  chromate  or  chromic  acid,  osmic  acid, 
alcohol,  and  formalin.  The  formulas  for  the  same  will  be 
found  on  pages  496  and  499. 

Hardening. — Since  all  the  fixing  agents  coagulate  living 
cytoplasm,  they  are  also  to  a  greater  or  less  extent  hardening 
31 


482  APPENDIX   CHAPTER  II 

agents.,  and  after  fixiii^r  tissues  may  be  handled  with  less 
danger  of  disturbing  the  relation  of  the  struetural  elements. 
Some  fixing  agents,  espeeially  chromic  fluids,  may  be  con- 
tinued in  their  action  as  hardening  agents  until  the  tissue  has 
attained  the  proper  consistency  for  sectioning,  but,  as  a  rule, 
it  is  necessary  to  use  other  agents  for  this  purpose.  In  all 
cases  the  fixhig  agent  must  he  thoroughly  trashed  out  of  the 
tissue  before  the  process  is  continued.  Alcohol  is  the  uni- 
versal hardening  agent,  and  at  the  same  time  it  removes  the 
water  from  the  tissue.  In  carrying  tissues  from  water  to 
alcohol  several  grades  must  always  be  used,  and  the  more 
delicate  the  tissue  the  more  gradual  must  be  the  changes. 
If  a  piece  of  tissue  is  taken  from  water  and  placed  in  95 
per  cent,  alcohol,  the  diffusing  currents  will  be  so  strong 
as  to  disturb  structure  and  at  the  same  time  the  hardening 
action  is  so  energetic  as  to  produce  shrinkage.  From  water  a 
tissue  should  never  be  placed  in  alcohol  stronger  than  70 
per  cent.,  where  it  should  be  allowed  to  remain  for  twenty- 
four  hours.  From  70  per  cent,  it  may  be  taken  to  95  per 
cent,  for  the  same  length  of  time,  and  from  95  per  cent,  to 
absolute,  which  will  entirely  remove  the  water  and  prepare 
the  tissue  for  embedding.  If  the  tissue  is  very  delicate, 
it  should  be  placed  in  water,  then  in  50  per  cent,  alcohol,  and 
carried  through  in  grades  of  10  per  cent,  to  95  per  cent. 

Embedding. — In  order  to  cut  thin  sections  of  tissue  the 
piece  must  be  surrounded  and  infiltrated  with  some  firm 
substance  which  will  not  only  support  the  entire  piece,  but 
will  soak  through  the  tissue,  filling  all  intercellular  spaces 
and  supporting  the  individual  structural  elements.  At  the 
same  time  the  embedding  material  is  used  to  fasten  the  tissue 
firmly  to  a  block  of  fiber  or  wood  which  can  be  grasped  in 
the  clamp  of  the  sectioning  machine.  Two  kinds  of  material 
are  used  for  this  purpose.  Substances  that  are  fluid  when 
warm,  and  solid  when  cold,  as  paraffin,  or  substances  which 
may  be  dissolved  in  volatile  liquid  and  are  solidified  by  evapo- 
ration, as  celloidin.  In  both  of  these  methods  the  substances, 
as  a  rule,  are  either  oily  or  insoluble  in  water,  and  therefore 
the  tissue  must  be  thoroughly  dehydrated — that  is,  have  all 


THE   THEORY  OF  HISTOLOGICAL   TECHNIQUE     483 

the  water  removed  from  it  before  it  is  placed  in  the  embed- 
ding material.  To  accomplish  this  there  should  be  at  least 
one  change  of  absolute  alcohol.  From  the  absolute  alcohol 
the  tissue  should  be  placed  in  a  fluid  which  is  a  solvent  for 
the  embedding  material,  so  that  it  will  penetrate  the  tissue 
more  perfectly  and  rapidly.  Heat  is  always  injurious  to  the 
tissue,  and  in  embedding  in  paraffin,  therefore,  the  tissue 
should  be  kept  in  the  melted  paraffin  for  the  shortest  possi- 
ble time  and  paraffin  of  as  low  a  melting  point  as  is  consist- 
ent with  sufficient  hardness  for  cutting  should  be  used.  In 
embedding  by  evaporation  the  evaporation  should  not  be  too 
rapid  or  the  shrinkage  will  be  increased.  Tissues  may  be 
kept  blocked  and  ready  to  cut  for  a  long  time,  but  as  a 
general  principle  the  shorter  the  time  the  more  perfect  will 
be  the  specimen. 

Sectioning. — For  sectioning  some  sort  of  machine  is 
necessary,  and  many  kinds  have  been  designed,  the  general 
principles  of  which  are  all  the  same.  They  consist  of  a 
clamp  which  holds  the  knife  and  a  clamp  which  holds  the 
specimen,  and  can  be  adjusted  in  such  a  way  as  to  bring  the 
specimen  in  proper  relation  to  the  knife.  The  position  of 
the  specimen  is  advanced  by  a  micrometer  screw  so  that 
sections  of  any  desired  thickness  may  be  sliced.  The  delicate 
part  of  this  machine  is  the  micrometer  screw.  The  essential 
to  the  success  of  its  working  is  the  sharpness  of  the  razor, 
and  for  such  specimens  as  decalcified  bone  the  razor  must 
be  heavy  and  strong,  so  that  the  edge  will  not  spring  in 
cutting  the  hard  tissue. 

Staining. — The  detail  of  staining  process  will  be  described 
in  the  next  chapter,  but  it  must  be  remembered  that  stains, 
as  a  rule,  are  water  solutions  and  the  sections  must  be  carried 
through  the  grades  of  alcohol  to  water  before  they  are  ready 
for  the  stain.  After  staining  they  must  be  carried  back 
through  the  grades  of  alcohol,  so  as  to  remove  the  water 
entirely  before  they  can  be  mounted  in  balsam,  which  is 
not  soluble  in  water. 

Mounting. — Except  in  serial  work,  but  one  specimen  should 
be  placed  on  a  slide,  and  this  should  be  in  the  centre,  leaving 


484  APPENDIX  CHAPTER  II 

room  at  either  end  for  a  label.  In  serial  work  the  sections 
may  be  placed  at  one  end  of  the  slide,  preferably  the  left 
hand,  leaving  room  at  the  right  for  one  label. 

Labelling. — Nothing  in  histological  technique  is  more  impor- 
tant than  labelling,  especially  in  all  research  work.  Through 
every  step  of  the  process  the  specimen  must  be  kept  track 
of,  and  a  mixing  of  labels  may  spoil  months  of  work.  A 
laboratory  notebook  containing  a  record  of  all  material 
and  work  should  always  be  on  the  tables.  I  have  found  a 
system  of  date  and  number  convenient.  For  instance,  on 
June  4  a  number  of  specimens  are  dissected  out;  in  the  note- 
book the  record  of  the  source  of  the  tissue  is  made;  the 
first  piece  is  placed  in  a  bottle  of  fixing  fluid  and  the  bottle 
labelled  6-4-1911,  No.  1;  the  second,  6-4-1911,  No.  2,  and 
so  on.  In  the  notebook  the  description  of  each  block  and 
the  date  and  the  hour  when  it  was  placed  in  the  fluid  is 
recorded.  In  this  way  the  tissue  may  be  carried  clear 
through  recording  each  step  in  the  process,  and  when  it  is 
sectioned  and  mounted  we  can  follow  its  history  in  the 
notebook.  Every  slide  should  be  labelled  first  with  the 
date  and  the  block  number  so  as  to  follow  its  technique; 
second,  the  name  of  the  tissue,  and  third,  the  kind  of  stain- 
ing. This  should  be  placed  on  the  right  hand  label,  leaving 
the  left  hand  label  for  index  and  file  number  if  the  section 
is  preserved. 

Indexing  and  Filing. — Many  beginners  make  the  mistake 
of  not  indexing  and  filing  their  slides.  They  think  because 
they  have  only  a  few,  that  they  can  easily  find  anything 
they  want,  and  that  they  will  wait  until  they  have  a  larger 
number  before  they  begin  a  system,  but  when  a  large 
number  have  piled  up  they  can  never  find  time  to  go  back 
and  arrange  them  as  they  should  be.  And  only  one  who 
has  failed  in  this  way  knows  the  annoyance  of  looking 
through  hundreds  of  slides  to  find  one  that  he  knows  he  has 
someplace. 


APPENDIX  CHAPTER  III 


GENERAL  HISTOLOGICAL  METHODS 

Fixing. — As  has  been  seen  from  the  preceding  chapter, 
fixing  is  the  first  and  one  of  the  most  important  steps  in  all 
histological  methods.  No  degree  of  care  in  the  latter  steps 
can  make  up  for  any  imperfection  in  it.  As  a  general 
statement  all  fixing  agents  have  advantages  and  disadvan- 
tages so  that  in  research  work  several  should  be  tried  and 
their  results  compared.  For  class-room  work,  however, 
minute  details  are  not  so  important.  Certain  general 
principles  may  be  stated.  Bichloride  of  mercury  is  espe- 
cially adapted  to  the  fixing  of  epithelium  of  the  mucous 
membrane.  It,  however,  does  not  penetrate  rapidly,  and 
small  pieces  must  be  used.  Crystals  are  liable  to  form  in 
the  tissue,  and  special  precautions  must  be  taken  for  their 
removal.  Flemming's  and  Zenker's  fluids  and  the  fluids 
containing  osmic  acid  are  used  chiefly  in  research.  For 
classwork  the  author  uses  Miiller's  fluid  and  Miiller's  fluid 
and  formalin  almost  entirely.  Stains  are  apt  to  work  better 
after  chromic  flxing  fluids.  The  formulas  for  several  of  the 
best  fixing  agents  with  directions  for  their  use  are  found  in 
the  last  chapter. 

Washing. — Except  for  special  purposes,  fixing  fluids  are 
washed  out  of  the  tissues  in  running  water,  and  they  should 
be  thoroughly  removed.  For  this  purpose  the  author  has 
made  a  galvanized  iron  tank  in  which  a  gauze  tray  divided 
into  small  gauze  compartments  is  suspended.  The  water 
is  brought  into  the  tank  through  a  rubber  tube  with  the 
mouth  resting  on  the  bottom,  and  leaves  through  a  spout 
at  the  top  to  which  another  tube  can  be  attached.    In  this 


APPENDIX  ('II AFTER  III 

way  a  lar^e  number  of  specimens  can  be  washed  at  once 
and  their  identity  followed. 

Preserving  Tissues. — After  washing,  the  tissues  should  be 
carried  through  the  grades  of  alcohol,  and  may  be  preserved 
for  a  considerable  time  in  SO  per  cent,  alcohol,  but  it  should 
be  changed  occasionally. 

Choice  of  Sectioning  Methods. — The  choice  between  paraffin 
and  celloidin  for  embedding  depends  upon  the  character 
of  the  section  desired  and  the  nature  of  the  tissue.  Small 
objects  and  those  of  delicate  structure,  such  as  embryos, 
dental  pulps,  etc.,  are  best  sectioned  in  paraffin.  Large 
pieces  and  blocks  containing  tissues  of  different  densities 
are  more  easily  cut  in  celloidin.  Paraffin  can  be  cut  much 
thinner  than  celloidin,  and  is  therefore  preferable  for  the 
minute  study  of  cell  structures  with  the  high  power.  Cel- 
loidin sections  are  more  easily  stained  and  are  easier  han- 
dled and  therefore  preferable  for  the  study  of  the  arrange- 
ment of  tissues  with  low  powers.  The  author  prefers 
celloidin  sections  for  classwork  whenever  possible. 

Embedding  in  Paraffin. — Tissues  fixed  and  washed  are  taken 
from  80  per  cent,  alcohol  and  placed  in  95  per  cent,  for  twenty- 
four  hours,  then  in  absolute  alcohol  for  the  same  length  of 
time,  and  the  absolute  alcohol  should  be  changed  once  duiing 
this  period,  from  absolute  alcohol  to  xylol,  in  which  the  tissue 
should  remain  until  it  is  clear  and  translucent.  The  time 
in  xylol  should  be  as  short  as  possible,  as  it  has  a  harden- 
ing action.  From  xylol  it  is  placed  in  a  solution  of  paraffin 
in  xylol,  and  from  this  to  soft  paraffin  in  the  paraffin  oven, 
at  a  temperature  of  not  over  52°  or  53°  C.  In  this  it  should 
remain  from  one-half  to  six  hours,  when  it  is  transferred  to 
hard  paraffin  in  the  oven  for  the  same  length  of  time.  The 
time  in  the  oven  should  always  be  as  short  as  is  consistent 
with  a  perfect  infiltration.  After  sufficient  time  in  hard 
paraffin  the  tissue  is  blocked  in  the  following  way:  A  mould 
is  made  by  placing  L-shaped  pieces  of  metal  together  on 
a  flat  slab.  These  are  manufactured  for  the  purpose. 
Melted  paraffin  is  poured  in  the  mould  and  the  tissue 
arranged  in  it,  placing  it  so  that  the  sections  will  cut  in  the 


GENERAL  HISTOLOGICAL  METHODS  487 

direction  desired.  A  film  of  paraffin  will  harden  at  once  on 
the  slab  and  the  tissue  can  be  placed  very  nicely  with  the 
needles.  As  soon  as  a  film  has  formed  over  the  surface  the 
slab  with  the  mould  should  be  immersed  in  cold  water,  so  as 
to  harden  the  paraffin  as  quickly  as  possible.  When  cold, 
sections  may  be  cut  at  once  or  the  block  may  be  preserved 
in  a  pasteboard  carton  properly  labelled.  As  a  rule,  paraffin 
sections  should  be  cut  as  soon  as  possible. 

Paraffin. — The  paraffin  for  embedding  sections  must  be 
of  the  best  quality.  That  prepared  for  this  purpose  by 
Griibler  is  preferable.  It  should  be  of  two  grades,  that  melt- 
ing at  45°  C,  and  that  melting  at  54°  C.  The  hard  paraffin 
is  mixed  with  the  softer,  so  as  to  give  a  melting  point  at 
about  52°.  In  winter  softer  paraffin  should  be  used  than  in 
summer,  as  the  cutting  quahty  depends  upon  the  adjust- 
ment of  the  paraffin  to  the  temperature  of  the  room.  If  the 
paraffin  is  too  hard  the  sections  are  liable  to  tear  and  curl; 
if  it  is  too  soft,  the  structure  of  the  tissue  will  be  disturbed 
in  cutting.  Perfect  infiltration  is  always  necessary  for  good 
sections.  Chloroform  or  oil  of  cedar  may  be  substituted  for 
xylol  in  this  process.  Xylol  is  most  rapid,  but  has  some 
disadvantages  in  its  action  on  the  tissues,  especially  if  left 
too  long. 

Cutting  Paraffin  Sections. — If  the  specimen  has  been  placed 
at  one  end  of  the  block,  the  other  end  of  the  paraffin  may 
be  clamped  in  the  microtome.  If  the  piece  is  too  small, 
it  should  be  fastened  to  a  block  of  vulcanized  fiber  with 
melted  paraffin  and  the  fiber  block  clamped  in  the  specimen 
holder.  With  a  sharp  scalpel  the  excess  of  paraffin  around  the 
specimen  should  be  trimmed  off,  leaving  the  block  in  a 
rectangular  form.  The  microtome  knife  is  placed  at  right 
angles  to  the  microtome  bed,  and  the  side  of  the  block 
should  be  parallel  with  the  blade.  The  specimen  should 
be  brought  up  just  to  the  edge  and  the  first  section  cut. 
The  knife  should  be  moved  with  a  quick,  sharp  motion,  as  par- 
affin sections  are  chopped  when  the  knife  is  in  this  position. 
The  knife  is  pushed  back,  the  block  lifted  with  the  micro- 
meter screw  so  as  to  give  a  section  of  the  proper  thickness, 


488  APPENDIX  CHAPTER  III 

and  the  second  section  cut.  If  the  paraffin  is  of  the  proper 
consistency  and  the  block  has  been  properly  trimmed,  the 
edge  of  the  second  section  will  stick  to  the  first  and  the 
sections  stretch  out  over  the  knife  in  a  ribbon.  The  ribbons 
may  be  transferred  to  a  piece  of  clean  white  paper  and  com- 
plete series  of  sections  cut.  When  series  are  not  required 
larger  specimens  are  often  cut  better  by  placing  the  blade 
of  the  knife  obliquely  and  drawing  it  with  a  slow,  even 
motion  through  the  block.  If  the  sections  show  a  tendency 
to  roll  up  when  the  corner  of  the  section  begins  to  curl 
over  the  edge  of  the  knife,  it  may  be  caught  with  the  tip  of 
a  camel's-hair  brush  and  so  section  after  section  transferred 
to  the  paper.  Paraffin  sections  should  cut  at  a  thickness  of 
from  seven  to  ten  microns,  but  sections  as  thin  as  one  micron 
may  be  cut  from  small  blocks  under  ideal  conditions. 

Handling  of  Paraffin  Sections. — For  staining,  paraffin  sec- 
tions must  be  fastened  to  the  slide  or  cover-glass.  If  a  few 
sections  are  to  be  cut  the  slide  is  preferable;  if  man}^  sections, 
as  in  the  preparation  of  class  work,  square  cover-glasses 
should  be  used.  In  either  case  the  glass  must  be  absolutely 
clean.  A  stock  of  perfectly  clean  slides  and  cover-glasses 
should  always  be  kept  on  hand  (see  p.  496).  A  thin  film 
of  albumin  fixative  is  spread  upon  the  glass;  this  film  must 
be  as  thin  as  possible.  The  best  way  to  spread  it  is  to  put 
a  drop  of  fixative  on  a  glass  slab  or  an  ordinary  slide,  touch 
the  edge  of  the  drop  with  the  end  of  the  little  finger  and 
spread  it  over  the  cover-glass,  wiping  off  all  that  can  be 
removed  with  the  finger.  Lay  the  cover-glasses  film  side  up 
on  a  piece  of  paper  until  the  required  number  have  been  pre- 
pared. As  each  section  is  cut  it  is  laid  on  a  cover-glass, 
straightened,  and  pressed  dow^n  with  a  camel's-hair  brush. 
If  the  sections  curl  or  wrinkle  they  should  be  floated  on 
water  warmed  just  enough  to  soften  the  paraffin  but  not 
melt  it.  As  each  section  is  cut  it  should  be  dropped  on  the 
top  of  the  water,  where  it  will  straighten  out.  When  a 
number  have  been  placed  on  the  surface  of  the  water  they 
may  be  picked  up  by  holding  the  cover-glass  in  the  point 
of  the  pliers  and  slipping  it  underneath  the  section  and 


GENERAL  HISTOLOGICAL  METHODS  489 

lifting  it  as  on  a  section  lifter.  The  water  is  drained  off  and 
the  cover-glass  placed  in  the  groove  of  the  tray  of  a  IMoore's 
staining  dish/  shown  in  Fig.  350.  Each  tray  will  hold  about 
thirty  cover-glasses.  They  must  now  be  thoroughly  dried 
by  leaving  them  over  night  at  room  temperature  or  for  a 
shorter  time  in  a  warm  oven,  which  should  not  be  hot  enough 
to  melt  the  paraffin.  When  dry,  each  cover-glass  should 
be  picked  up  in  the  pliers  and  passed  quickly  through  the 
middle  of  a  Bunsen  flame,  so  as  to  coagulate  the  albumin, 
or  they  may  all  be  fixed  at  once  in  an  oven.  Heat  that  will 
just  melt  the  paraffin  will  coagulate 
the  albumin  and  hold  the  section  on  Fig  Soo 

the  glass.     By  means  of  a  little  wire  , — ^ --.^ 

basket  the  tray  with  the  thirty  cover-       (1^^^^ 
glasses  may    now   be   carried   from      X^^J^F^ 
one  dish  to  another  through  the  fol-        ^^^--^  *-   ^ 
lowing  necessary  reagents.     First,  a  ^     ^==^^^ 

minute  or  two  in  xylol  to  remove  the  Morris  staining  dish. 

paraffin;  then  absolute  alcohol,  then 

70  per  cent.;  then  water;  Delafield's  hematoxylin  for  five 
minutes;  distilled  water  to  wash  off  the  stain;  acid  alcohol 
(70  per  cent,  alcohol  to  which  2  or  3  drops  of  hydrochloric 
acid  has  been  added  to  every  100  c.c.  of  alcohol);  again 
washed  in  tap  water  to  remove  and  neutralize  the  acid 
(some  prefer  alcohol  to  which  a  few  drops  of  ammonia  have 
been  added);  70  per  cent,  alcohol;  eosin  for  thirty  seconds; 
70  per  cent,  alcohol,  then  95  per  cent.,  then  absolute,  and 
finally  xylol.  From  the  xylol  the  sections  may  be  mounted 
or  given  out  to  the  class.  For  class  work  a  student  brings 
to  the  desk  a  clean  slide  with  a  drop  of  balsam  on  the 
centre  and  receives  a  section. 
Summary  of  Paraffin  Method. — 

Tissues  in  80  per  cent,  alcohol. 

95  per  cent,  alcohol,  twenty-four  hours. 

Absolute  alcohol  (changed  once),  twenty-four  hours. 

Xylol,  one-half  to  six  hours. 

1  These  are  manufactured  by  Bausch  &,  Lomb. 


490  APPENDIX  CHAPTER  III 

X3I0I  and  paraffin,  one-half  honr. 

Soft  paraffin,  one-half  to  six  hours. 

Hard  paraffin,  one  to  six  hours. 

Block. 

Section. 

Fix  on  glass. 

Heat. 

Xylol,  one  minute. 

Absolute  alcohol,  one  minute. 

95  per  cent,  alcohol,  same. 

70  per  cent,  alcohol,  same. 

Distilled  water. 

Hematoxylin,  five  to  ten  minutes. 

Tap  water. 

Acid  alcohol. 

Tap  water  or  ammonia  alcohol. 

70  per  cent,  alcohol. 

Eosin,  thirty  seconds. 

70  per  cent,  alcohol. 

95  per  cent,  alcohol. 

Absolute  alcohol. 

Xylol. 

Mount  in  balsam. 

Label. 
Celloidin  Method. — Tissues  fixed  and  washed  are  taken  from 
80  per  cent,  alcohol  and  placed  in  95  per  cent,  for  twenty-four 
hours;  then  in  absolute  alcohol  for  the  same  length  of  time, 
changing  the  alcohol  once.  Then  into  a  mixture  of  absolute 
alcohol  and  ether  for  twenty-four  hours,  from  this  into  a 
thin  solution  of  celloidin,  in  which  they  should  remain  for 
from  two  days  to  a  week.  From  the  thin  solution  they  should 
be  placed  in  a  thick  celloidin  solution,  about  the  consistency 
of  syrup,  for  the  same  length  of  time.  The  tissues  may  be 
kept  in  the  celloidin  solution  indefinitely  without  injury,  and 
if  the  tissue  is  difficult  to  infiltrate  it  may  be  of  advantage 
to  leave  them  in  these  solutions  for  weeks  or  months.  In 
this  case  the  bottles  must  of  course  be  perfectly  corked  to 
prevent  evaporation. 


GEXERAL  HISTOLOGICAL  METHODS  491 

Blocking  of  Celloidin  Material. — There  are  several  methods 
for  blocking  celloidin  materials,  of  which  the  author  prefers 
the  following:  Thick  celloidin  is  poured  into  a  stender  dish 
or  a  small  Petrie  dish  until  there  is  enough  to  abundantly 
cover  the  specimens,  which  are  arranged  on  the  bottom  of 
the  dish.  A  match  or  bit  of  cork  is  placed  under  the  edge 
of  the  cover  so  as  to  allow  slow  evaporation.  In  a  day  or 
two  the  celloidin  will  attain  the  consistence  of  a  thick  jelly. 
A  knife  is  now  passed  around  each  tissue  and  the  celloidin 
containing  the  specimen  lifted  out,  and  the  excess  of  celloidin 
is  trimmed  away.  A  vulcanized  fiber  block  has  one  surface 
dipped  into  the  thick  celloidin  and  the  specimen  arranged 
upon  it.  Thick  celloidin  is  now  added  to  surround  and  cover 
the  tissue  with  its  adherent  celloidin.  As  soon  as  this  is 
hardened  so  as  to  form  a  film  it  is  dropped  into  80  per  cent, 
alcohol  to  harden  the  entire  mass.  In  this  it  must  remain 
at  least  twenty-four  hours  before  it  can  be  sectioned.  Tissues 
embedded  in  celloidin  may  be  kept  for  years  in  80  per  cent, 
alcohol  blocked  and  ready  to  cut  without  great  injury  to 
the  tissue. 

Celloidin  solutions  for  embedding  should  be  kept  in  two 
grades  and  labelled  "thick"  and  "thin"  celloidin.  The 
latter  should  be  quite  fluid,  the  former  about  a  syrup 
consistence.  Scherring's  celloidin  is  furnished  in  two 
forms,  in  shreds  and  granules.  The  former  will  dissolve 
more  rapidly.  About  half  an  ounce  is  placed  in  a  large- 
mouthed  bottle,  and  a  mixture  of  equal  parts  of  absolute 
alcohol  and  ether  added.  It  dissolves  slowly  and  should 
be  shaken  frequently.  When  this  solution  is  sufficiently 
thick,  part  may  be  poured  into  another  bottle  and  diluted 
with  sufficient  absolute  alcohol  and  ether  for  the  thin  solution, 
while  the  thicker  portion  is  poured  into  a  bottle  for  the  thick 
solution,  and  absolute  alcohol  and  ether  may  be  added  to 
the  stock  bottle  to  dissolve  the  residue.  When  blocking 
tissues  as  described  above  the  trimmings  are  dropped  back 
into  the  stock  bottle. 

Cutting  Celloidin  Sections. — The  fiber  block  is  clamped  in 
the  specimen  holder  and  adjusted.    The  knife  is  set  diago- 


492  APPENDIX  CHAPTER  III 

nally  so  as  to  cut  with  a  drawing  motion,  and  both  the  knife 
and  the  block  are  kept  flooded  with  80  per  cent,  alcohol. 
The  sections  may  be  allowed  to  pile  up  on  the  knife,  and  after 
eight  or  ten  are  cut  they  are  slid  off  with  a  camel's-hair 
brush  on  to  a  section  lifter  and  transferred  to  80  per  cent, 
alcohol,  in  which  they  may  be  kept  for  some  time. 

Staining  Celloidin  Sections. — For  transferring  celloidin  sec- 
tions the  most  convenient  thing  is  a  small  tea-strainer  with  a 
handle.  These  may  be  got  for  a  few  cents  at  any  hardware 
store.  By  means  of  this  the  sections  are  transferred  to  70  per 
cent,  alcohol,  from  this  to  distilled  water,  and  are  stained 
from  five  to  ten  minutes  in  Delafield's  hematoxylin.  The 
stain  is  then  washed  off  with  tap  water,  destained  with  acid 
alcohol,  w^ashed  in  tap  water  or  ammonia  alcohol,  stained 
thirty  seconds  in  eosin,  washed  with  70  per  cent,  alcohol, 
from  this  to  95  per  cent.,  in  which  they  should  be  given  two 
or  three  changes.  From  this  they  are  transferred  to  beech- 
wood  creosote  or  some  other  clearing  agent  (see  p.  503), 
and  in  this  they  may  be  kept  until  they  are  ready  to  mount 
or  to  be  given  out  to  the  class.  For  class  w^ork  the  student 
brings  to  the  desk  a  clean  slide,  and  a  section  is  placed  upon 
the  centre  of  it.  After  blotting  off  the  excess  of  oil  he  adds 
a  drop  of  balsam,  covers  with  a  cover-glass,  and  labels  the 
specimen. 

Summary  of  Celloidin  Method. — 

Tissues  in  80  per  cent,  alcohol. 

95  per  cent,  alcohol,  twenty-four  hours. 

Absolute  alcohol,  changed  twice,  twenty-four  hours. 

Absolute  alcohol  and  ether,  twenty-four  hours. 

Thin  "celloidin,  two  days  to  a  week. 

Thick  celloidin,  the  same. 

Evaporate. 

Block. 

80  per  cent,  alcohol  to  harden  or  store. 

Sections  cut  in  80  per  cent,  alcohol. 

70  per  cent,  alcohol,  one  minute. 

Distilled  water. 

Hematoxylin,  five  to  ten  minutes. 


GENERAL  HISTOLOGICAL  METHODS  493 

Tap  water. 

Acid  alcohol. 

Tap  water  or  ammonia  alcohol. 

70  per  cent,  alcohol. 

Eosin  one  minute. 

70  per  cent,  alcohol  to  wash. 

95  per  cent,  alcohol,  changed  twice. 

Creosote. 

Mount  in  balsam. 

Label. 
Serial  Sections  with  Celloidin. — It  is  difficult  to  cut  series 
of  sections  with  the  celloidin  method.  The  simplest  process 
and  one  used  with  success  is  to  carry  the  sections  in  order 
from  the  knife  to  the  slide,  arranging  three  or  four  at  one 
end  of  it  and  leaving  room  for  a  label.  Strips  of  porous 
tissue  paper  are  cut  the  proper  size  and  one  laid  over  the 
sections  to  hold  them  in  place.  A  thread  is  then  lightly 
wrapped  around  the  slide  and  paper,  when  they  may  be 
carried  through  the  necessary  agents  for  staining,  in  Naples 
jars.  After  they  are  cleared  the  paper  is  removed,  the  excess 
of  the  oil  blotted  off,  the  balsam  put  upon  the  section  and 
covered  with  a  long  cover-glass. 


SPECIAL  METHODS 

Dental  Pulp. — The  unerupted  premolars  from  a  young 
sheep  furnish  excellent  material  for  the  study  of  the  dental 
pulp.  The  jaws  of  sheep  slaughtered  for  spring  lamb  can 
be  easily  obtained  from  the  stockyards,  and  while  still  warm 
are  placed  in  JNIiiller's  fluid  and  formalin,  in  which  they  are 
taken  to  the  laboratory.  The  temporary  incisors  are  still 
in  place  and  may  be  used  for  peridental  membrane  material. 

With  the  bone  forceps  the  cortical  plate  is  removed  and  the 
unerupted  teeth  dissected  from  their  crypts.  By  grasping 
the  base  of  the  dental  papillae  with  the  pliers  the  pulp  may 
be  pulled  out  of  the  dentin.  They  should  then  be  replaced 
in  Muller's  fluid  and  formalin  for  twenty-four  hours  when 


494  APPENDIX   CHAPTER  III 

tlu'v  may  be  carried  through  the  usual  process,  embedded 
ill  ])araffin,  and  sectioned. 

Human  Pulps. — By  the  cooperation  of  the  extracting  room 
human  })ulps  for  liistoh)gical  work  may  be  obtained.  As 
soon  as  extracted  the  tooth  should  be  wrapped  in  a  gauze 
napkin,  placed  in  the  jaws  of  a  heavy  vise,  which  is  carefully 
tightened  until  the  tooth  cracks.  The  same  thing  may  be 
accomplished  by  a  heavy  hammer  on  an  anvil.  A  few  trials 
of  this  will  enable  one  to  crack  the  tooth  so  that  the  pulps 
may  be  easily  remo\'ed  without  injury.  The  cracked  tooth 
is  put  in  Miiller's  fluid  and  formalin  for  twenty-four  hours, 
when  the  pieces  of  dentine  are  removed  and  the  pulp  care- 
fully lifted  out  of  the  pulp  chamber.  It  is  then  carried 
through  the  regular  process,  embedded  in  paraffin,  and 
sectioned.  If  the  teeth  are  not  perfect  clinical  history  should 
be  noted. 

Periosteum. — Young  kittens  that  have  not  attained  their 
full  growth  may  be  used  for  this  purpose.  The  bone  should 
be  very  carefully  dissected  so  as  not  to  injure  the  periosteum 
and  then  sawed  in  pieces,  using  a  fine  metal  saw.  It  is  usually 
best  simply  to  saw  it  in  two  at  the  middle  of  the  shaft  and 
to  fix  it  in  Miiller's  fluid  and  formalin.  After  fixing  and 
washing,  it  should  be  cut  in  small  pieces  and  decalcified 
in  2  to  5  per  cent,  nitric  acid.  A  comparatively  large  volume 
of  acid  should  be  used  and  a  pad  of  cotton  placed  in  the 
lower  half  of  the  bottle,  or  the  tissue  suspended  by  a  thread. 
It  is  best  to  change  the  acid  once  a  day.  Decalcification 
may  require  from  two  days  to  a  week,  and  should  be  tested 
by  passing  sharp  needles  through  the  tissues.  As  soon  as 
decalcified  the  tissue  should  be  washed  for  twenty-four 
hours  in  running  w^ater,  carried  through  the  grades  of  alcohol, 
and  embedded  in  celloidin.  The  sections  should  be  cut  at 
right  angles  to  the  shaft. 

Peridental  Membrane. — For  class  work  the  peridental  mem- 
branes of  sheep  are  the  best  for  study,  as  their  fibers  are  large 
and  their  direction  easily  observed.  They  are  much  better 
than  those  of  either  cat  or  dog,  in  which  the  fibers  are  much 
finer  and  the  bone  more  dense.    The  jaws  are  brought  from 


GENERAL  HISTOLOGICAL  METHODS  495 

the  stockyards  in  ]\Iiiller's  fluid  and  formalin,  the  crowns 
broken  ofl'  at  the  level  of  the  gum  so  as  to  expose  the  pulp 
chamber,  and  the  jaws  sawed  through  so  as  to  leave  two 
teeth  in  each  block,  after  which  they  are  replaced  in  Miiller's 
fluid  and  formalin  for  two  days,  decalcified  in  nitric  acid, 
and  thoroughly  washed.  They  may  now  be  cut  into  small 
blocks  for  transverse  sections  and  embedded  in  celloidin. 
Embryological  Material. — For  the  study  of  the  tooth  germ 
in  class  work  embryo  pigs  of  all  ages  are  easily  obtained. 
The  entire  embryo  should  be  at  once  placed  in  Miiller's 
fluid  or  a  saturated  solution  of  picric  acid  and  water.  In 
^Miiller's  fluid  they  should  remain  a  week;  in  picric  acid, 
forty-eight  hours.  After  fixing,  the  heads  are  cut  ofi",  thor- 
oughly washed,  carried  through  the  grades  of  alcohol,  and 
embedded  in  paraffin. 


APPENDIX  CHAPTER  IV 


FIXING  AGENTS  AND  STAINING  SOLUTIONS 

Cleaning  of  Slides  and  Cover-glasses. — Slides  or  cover- 
glasses  on  which  paraffin  sections  are  to  be  mounted  must 
be  absolutely  clean.  They  should  be  dropped  in  strong 
sulphuric  acid  and  allowed  to  remain  a  few  minutes.  The 
acid  should  then  be  poured  off  and  thoroughly  removed  with 
water,  and  strong  acetic  acid  poured  on.  After  remaining 
a  few  minutes  wash  the  acid  off  thoroughly  and  wipe  from 
alcohol.    Keep  ready  for  use  in  a  clean  box. 

Meyer's  Fixative. — The  white  of  an  egg  is  chopped  with 
a  pair  of  scissors  and  filtered  through  muslin,  diluted  with 
an  equal  volume  of  glycerin,  and  a  little  sodium  oxalate 
added  to  prevent  decomposition. 

FIXING  AGENTS 

Flemming's  Solution. — A  good  solution  for  fixing  nuclear 
structures  is  the  chromic-acid  solution  of  Flemming: 

Parts. 

Osmic  acid,  1  per  cent,  aqueous  solution 10 

Chromic  acid,  1  per  cent,  aqueous  solution 25 

Glacial  acetic  acid,  1  per  cent,  aqueous  solution 10 

Distilled  water 55 

Small  pieces  are  fixed  in  a  small  quantity  of  the  fluid  for 
at  least  twenty-four  hours.  They  are  then  washed  for  the 
same  number  of  hours  in  running  water  and  passed  through 
50,  75,  and  80  per  cent,  each  twenty-four  hours  into  90 
per  cent,  alcohol. 


FIXIXG  AGEXTS  497 

A  stronger  solution  is  made  as  follows: 

Part3. 

Osmic  acid,  2  per  cent,  aqueous  solution 4 

Chromic  acid,  1  per  cent,  aqueous  solution 15 

Glacial  acetic  acid 1 

Fol's   Solution. — A  modification  of  Flemming's  solution: 

Parts. 

Osmic  acid,  1  per  cent  aqueous  solution 2 

Chromic  acid,  1  per  cent,  aqueous  solution 25 

Glacial  acetic  acid,  2  per  cent,  aqueous  solutioa 5 

Distilled  water 68 

Corrosive  Sublimate. — An  excellent  fixing  fluid  is  made  by 
saturating  distilled  water  with  corrosive  sublimate.  Small 
pieces  about  0.5  cm.  in  diameter  are  immersed  in  this  fluid 
for  from  three  to  twenty-four  hours,  then  washed  in  running 
water  for  twenty-four  hours,  and  then  transferred  into  70 
per  cent,  alcohol.  After  twenty-four  hours  the  tissues  are 
placed  in  SO  per  cent,  for  the  same  length  of  time  and  then 
preserved  in  90  per  cent.  It  often  occurs  that  after  changes  in 
temperature  crystals  of  sublimate  are  formed  on  the  surface 
or  in  the  interior  of  the  object.  For  their  removal  a  few 
drops  of  iodine  and  potassium  iodide  are  added  to  the  alcohol 
(P.  Mayer).  It  is  a  matter  of  indifference  whether  the  70 
per  cent.,  SO  per  cent.,  or  90  per  cent,  alcohol  is  thus  iodized. 
In  future  treatment  of  the  object,  as  well  as  in  sectioning, 
any  such  crystals  of  sublimate  will  not  be  found  to  be  a 
hindrance.  In  the  case  of  delicate  objects  it  is  better  to 
undertake  their  removal  after  sectioning  by  adding  iodine  to 
the  absolute  alcohol  then  used. 

Acetic  Sublimate  Solution. — An  excellent  solution  specially 
used  for  embryonic  tissues  and  for  organs  containing  only 
a  small  quantity  of  connective  tissue.  To  a  saturated 
aqueous  solution  of  sublimate,  5  to  10  per  cent,  of  glacial 
acetic  acid  is  added.  After  remaining  two  to  three  hours 
or  more  in  this  solution,  the  objects  are  transferred  to  35 
per  cent,  alcohol  and  then  passed  through  the  higher  grades 
of  alcohol. 
32 


498  APPENDIX   CHAPTER  IV 

Picric  Acid. — Small  and  medium-sized  objects  (up  to 
1  c.c.)  are  fixed  in  twenty-four  hours  in  a  saturated  aqueous 
solution  of  picric  acid  (about  0.75  per  cent.).  Objects  of 
considerable  size  may  be  left  in  this  solution  for  weeks  with- 
out detriment.  The  tissues  are  then  transferred  to  70  or 
80  per  cent,  alcohol,  in  which  they  remain  until  the  alcohol 
is  not  colored  by  the  picric  acid.  Instead  of  a  pure  solution 
of  picric  acid,  the  picrosulphuric  acid  of  Kleinenberg,  or 
the  picric  acid  of  P.  INIayer  may  be  used.  Picrosulphuric 
acid  is  made  as  follows:  1  c.c.  of  concentrated  sulphuric 
acid  is  added  to  100  c.c.  of  a  saturated  aqueous  picric  acid 
solution.  Allow  this  to  stand  for  twenty-four  hours  and 
dilute  with  double  its  volume  of  distilled  w^ater.  The  picric 
acid  solution  is  made  by  adding  2  c.c.  of  pure  nitric  acid 
to  100  c.c.  of  saturated  picric  acid  solution.  Filter  after 
standing  for  twenty-four  hours. 

Chromic  Acid. — Chromic  acid  is  used  in  a  |  to  1  per  cent, 
aqueous  solution.  Small  pieces  are  fixed  for  twenty-four 
hours,  larger  ones  for  a  longer  time.  The  quantity  of  the 
fixing  fluid  should  equal  at  least  more  than  fifty  times  the 
volume  of  the  tissues  to  be  fixed.  After  fixing,  objects  must 
be  w^ashed  for  at  least  tw^enty-four  hours  in  running  water,' 
then  through  the  grades  of  alcohols,  and  preserved  in  80 
per  cent.  Two  to  3  drops  of  formic  acid  to  every  100  c.c. 
of  chromic  acid  solution  improve  their  fixing  properties. 

MuUer's  Fluid. — 

Potassium  bichromate 2  to  2.5  grams 

Sodium  sulphate 1  gram 

Water 100  c.c. 

This  solution  requires  a  long  time  for  fixing,  at  least 
several  weeks,  and  for  large  pieces  several  months.  During 
the  first  few  weeks  the  solution  should  be  changed  every 
three  or  four  days  and  later  once  a  week,  until  it  remains 
clear.  Tissues  should  be  thoroughly  washed  in  running 
water  at  least  tw^enty-four  hours.  For  some  special  purposes 
it  is  better  to  wash  in  alcohol.  Tissues  should  be  carried 
through  the  grades  and  preserved  in  80  per  cent,  alcohol. 


FIXING  AGENTS  499 

Wliile  tissues  are  in  ^Nliiller's  fluid  they  should  be  kept  in 
the  dark. 

Miiller's  Fluid  and  Formalin. — 

Muller's  fluid 100  c.c. 

Formalin 10  c.c. 

The  addition  of  formahn  to  Miiller's  fluid  greatly  hastens 
fixation.  It  is  an  excellent  agent  of  great  penetrating  power, 
and  tissues  stain  very  well  after  it.  Twenty-four  hours  will 
fix  tissues  of  ordinary  size,  though  they  may  be  left  longer 
without  damage.  Bone  fixed  too  long  in  formalin  is  liable 
to  be  hard  to  cut. 

Zenker's  Fluid. — 

Grams. 

Potassium  bichromate 2.5 

Sodium  sulphate .  1.0 

Corrosive  sublimate                                                                             .  5.0 

Glacial  acetic  acid    .      .                                                                      .5.0 
Water 100.0 

Add  the  glacial  acid  in  proper  proportion  to  the  quantity 
of  the  solution  to  be  used,  and  not  to  the  stock  solution. 
x'Vllow  the  tissues  to  remain  in  this  solution  for  from  six  to 
twenty-four  hours.  Then  wash  in  running  water  for  from 
twelve  to  twenty-four  hours  and  transfer  to  gradually 
concentrated  alcohol.  Crystals  of  sublimate  which  may  be 
present  are  removed  with  iodized  alcohol.  Zenker's  fluid 
penetrates  easily  and  fixes  nuclear  and  protoplasmic  struc- 
tures equally  well  without  decreasing  the  staining  qualities 
of  the  elements. 

Formalin. — Of  recent  years  formalin,  which  is  a  4  per  cent, 
solution  of  the  gas  formaldehyde  in  water,  has  been  much 
used  as  a  fixing  fluid.  ^Nlake  a  solution  by  adding  10  parts 
of  formalin  to  90  parts  of  water  or  normal  saline  solution. 
Small  pieces  of  tissue  should  remain  in  this  for  from  twelve 
to  twenty-four  hours,  larger  pieces  a  number  of  days  or 
weeks,  and  then  transfer  to  90  per  cent,  alcohol. 


500  APPENDIX   CHAPTER  IV 

STAINING  AGENTS 

Delafield's  Hematoxylin. — 

Hematoxylin  crj'stals 4  grams 

Absolute  alcohol 25  c.c. 

Ammonia  alum,  aqueous  solution 400  c.c. 

Methyl  alcohol 100  c.c. 

Glycerin 100  c.c. 

Dissolve  hematoxylin  crystals  in  absolute  alcohol  and  add 
to  the  alum  solution,  place  in  an  open  vessel  for  four  days, 
then  filter  and  add  the  methyl  alcohol  and  glycerin. 

Hemalum  (Mayer,  91). — One  gram  of  hematin  is  dissolved 
by  heating  in  50  c.c.  of  absolute  alcohol.  This  is  poured  into 
a  solution  of  50  grams  of  alum  in  1  liter  of  distilled  water 
and  the  whole  well  stirred.  A  thymol  crystal  is  added  to 
prevent  the  growth  of  fungus.  The  advantages  of  hemalum 
is  as  follows:  The  stain  may  be  used  immediately  after  its 
preparation,  it  stains  quickly,  never  overstaining,  especially 
when  diluted  with  water,  and  penetrates  deeply,  making  it 
useful  for  staining  in  bulk.  After  staining  sections  or  tissues 
are  washed  in  distilled  water. 

Safranin. — 

Safranin 1  gram 

Absolute  alcohol 10  c  c. 

Aniline  water 90  c.c. 

Aniline  water  is  prepared  by  shaking  up  5  c.c.  to  8  c.c.  of 
aniline  oil  in  100  c.c.  of  distilled  water  and  filtered  through  a 
wet  filter.  Dissolve  the  safranin  in  the  aniline  water  and  add 
the  alcohol.    Filter  before  using. 

Stain  sections  fixed  in  Flemming's  solution  for  twenty- 
four  hours  and  decolorize  with  a  weak  solution  of  hydro- 
chloric acid  in  absolute  alcohol  (1  to  1000).  After  a  varying 
period  of  time,  usually  only  a  few  minutes,  all  the  tissue 
elements  will  be  found  to  have  become  bleached,  only  the 
chromatin  of  the  nucleus  retaining  the  color. 


STAINING  AGENTS  501 

Methyl  Green.— Stains  very  quickly.  One  gram  is  dis- 
solved in  100  c.c.  of  distilled  water  to  which  25  c.c.  of  absolute 
alcohol  is  added.  Rinse  the  sections  in  water,  then  place  in 
70  per  cent,  alcohol  for  a  few  minutes,  transfer  to  absolute 
alcohol  for  a  minute,  etc. 

Hematoxylin. — Van  Gieson's  Acid  Fuchsin-Picric  Acid  Solu- 
tion.— Stain  in  any  of  the  hematoxylin  solutions,  and  after 
rinsing  sections  in  water  counterstain  in  the  following: 

Acid  fuchsin,  1  per  cent,  aqueous  solution 5  c.c. 

Picric  acid,  saturated  aqueous  solution 100  c.c. 

Dilute  with  an  equal  quantity  of  water  before  using.  The 
hematoxylin  stained  sections  remain  in  the  solution  from  one 
to  two  minutes,  are  then  rinsed  in  water,  dehydrated,  and 
cleared. 

Hematoxylin-Eosin.— Sections  already  stained  in  hema- 
toxylin are  placed  for  two  to  five  minutes  in  a  1  to  2  per 
cent,  aqueous  solution  of  eosin  or  in  a  1  per  cent,  solution 
of  eosin  in  a  60  per  cent,  solution  of  alcohol.  They  are  then 
washed  in  water  until  free  from  the  stain,  after  which  they 
remain  for  a  short  time  in  absolute  alcohol.  In  place  of  the 
eosin  solution  a  1  per  cent,  aqueous  solution  of  benzopurpurin 
may  be  used  for  the  following  solution  of  erythrosin  (Held). 

Erythrosin 1  gram 

Distilled  water 150  c.c. 

Glacial  acetic  acid 3  drops 

Silver  Nitrate  Method. — Especially  useful  for  staining  inter- 
cellular substances  of  epithelium,  endothelium,  and  meso- 
thelium,  and  the  ground  substance  of  connective  tissues. 
It  may  be  used  on  either  fresh  or  fixed  tissues,  fresh  tissue, 
however,  being  more  satisfactory.  Spread  the  tissues  to  be 
stained  in  thin  layers;  immerse  in  a  0.5  to  1  per  cent,  solution 
of  silver  nitrate  from  ten  to  fifteen  minutes;  rinse  in  distilled 
water  and  place  in  fresh  distilled  water  or  70  per  cent, 
alcohol  or  a  4  per  cent,  solution  of  formalin  and  expose  to 
direct  sunlight  until  they  assume  a  brown  color.  The  sun- 
light reduces  the  silver  in  the  form  of  fine  particles  which 


502  APPENDIX  CHAPTER  IV 

appear  black  on  being  examined  with  transmitted  light. 
The  preparations  thus  obtained  may  be  examined  in  glycerin 
or  dehydrated  and  mounted  in  balsam. 

Glycerin. — To  mount  in  glycerin  transfer  the  sections  from 
water  to  the  slide,  cover  with  a  drop  of  gl^xerin,  and  apply 
the  coverslip.  Sections  colored  with  a  stain  that  would  be 
injured  by  contact  with  alcohol  and  where  clearing  is  not 
especially  necessary  are  mounted  this  way. 

Farrant's  Gum  Glycerin. — In  place  of  pure  glycerin  the 
following  mixture  may  be  used: 

Gljcerin 50  c.c. 

Water .50  c.c. 

Gum  arabic  (powder)     .                                    .                        .50  grams 
Arsenous  acid 1  gram 

Dissolve  the  arsenous  acid  in  water.  Place  the  gum- 
arabic  in  a  glass  mortar  and  mix  it  with  the  water,  then  add 
the  gl^'cerin.  Filter  through  a  wet  filter  paper  or  through 
fine  muslin.  To  preserve  such  preparations  for  any  length 
of  time  the  cover-glasses  must  be  so  fixed  as  to  shut  off  the 
glycerin  from  the  air.  For  this  purpose  cements  or  varnishes 
are  used,  by  painting  over  the  edges  of  the  cover-glass. 
These  masses  adhere  to  the  glass,  harden,  and  fasten  the 
cover-glass  firmly  to  the  slide,  hermetically  sealing  the  object. 
Kronig's  is  one  of  the  best  formulas  for  varnish,  and  is  made 
as  follows:  Melt  2  parts  of  wax  and  stir  in  7  to  9  parts  of 
colophonium  and  filter  the  mass  hot.  Before  employing 
an  oil  immersion  lens  it  is  best  to  paint  the  edges  with  an 
alcoholic  solution  of  shellac. 

Silver  Nitrate. — In  thin  membranes  and  sections  the  vessel 
walls  can  be  rendered  distinct  by  silver  impregnation,  which 
brings  out  the  outlines  of  their  endothelial  cells.  This  may 
be  done  either  by  injecting  the  vessel  with  a  1  per  cent, 
solution  of  silver  nitrate,  or  with  a  0.25  per  cent,  solution  of 
silver  nitrate  in  gelatin.  This  method  is  of  advantage,  since 
after  hardening  the  capillaries  of  the  injected  tissues  appear 
slightly  distended.  Organs  thus  treated  can  be  sectioned, 
but  the  endothelial  mosaic  of  the  vessels  does  not  appear 
definitely  until  the  sections  have  been  exposed  to  sunlight. 


STAINING  AGENTS  503 

The  injections  of  lymph  channels,  lymph  vessels,  and  lymph 
spaces  is  usually  done  by  puncture.  A  pointed  cannula  is 
thrust  into  the  tissue  and  the  syringe  empties  by  a  slight 
but  constant  pressure.  The  injected  fluid  spreads  by  means 
of  the  channels  offering  the  least  resistance.  For  this  pur- 
pose it  is  best  to  use  aqueous  solution  of  Berlin  blue  or  silver 
nitrate,  as  the  thicker  gelatin  solutions  cause  tearing  of  the 
tissues. 

■  Clearing  Agents. — Clearing  agents  are  substances  of  high 
refracting  index,  mostly  oils,  which  are  used  to  displace 
alcohol  and  prepare  tissues  for  embedding  and  sections  for 
mounting  in  balsam. 

Clearing  agents  for  embedding  in  paraffin  must  be  miscible 
with  alcohol  and  solvents  for  paraffin.  They  are  called 
clearing  agents  because  the  tissues  become  translucent  and 
clear  in  them.  Xylol  is  the  most  rapid  and  probably  most 
used  agent.  It  has,  however,  a  hardening  action  on  the 
tissues,  especially  if  they  remain  too  long  in  it.  Pure  oil 
of  cedar  wood  when  free  from  turpentine  is  an  excellent 
agent.  Chloroform  has  been  largely  used  for  the  same 
purpose. 

Before  celloidin  sections  are  mounted  in  balsam  they 
must  be  cleared.  For  this  purpose  an  oil  that  will  mix  with 
95  per  cent,  alcohol  is  desirable,  as  absolute  alcohol  softens 
the  celloidin.  The  oil  used  must  not  dissolve  the  celloidin, 
and  should  not  dissolve  the  stain.  Beechwood  creosote  is 
an  excellent  agent,  and  has  been  largely  used.  It  clears 
sections  rapidly  from  95  per  cent,  alcohol.  Oil  of  bergamot 
is  an  excellent  agent,  also  oil  of  origanum;  but  in  the  latter 
the  oleum  origani  cretici  and  not  the  oleum  origani  gallici 
must  be  used.  A  mixture  of  equal  parts  of  oil  of  bergamot 
and  beechwood  creosote  has  been  used  satisfactorily,  and 
is  an  excellent  agent.  A  cheaper  mixture  is  made  of  equal 
parts  of  phenol,  oil  of  origanum,  and  oil  of  cedarwood. 


INDEX 


Absorptiox  of  roots  of  temporar}- 
teeth,  302 

Acetic    acid    and    sublimate    for 
fixing,  497 

Alveolar  process,  379 

Analog}',  22 

Attachment  of  teeth,  271 
by  ankylosis,  274 
in  fibrous  membrane,  272 
by  hinged  joint,  273 
by  insertion  in  a  socket,  277 


B 


Balsam,  463 

management  of,  for  grinding  sec- 
tions, 469 
Bichloride  of  mercury  for  fixing, 

497 
Blocking  celloidin  material,  491 
Bone,  247 

arrangement  of  lamellae  of,  252 
canaliculi  of,  249 
cancellous,  251,  254 
compact,  252 
corpuscles  of,  248 
decalcified,  442 
definition  of,  247 
formation  of,  255 

endochondrial,  255,  445 
endomembranous,  258 
growth  of,  260,  446 
Haversian  canals  of,  253 

system  of,  250 
influence   of   mechanical   condi- 
tions on,  380 
lacunae  of,  249 


Bone,  matrix  of,  247 

structural  elements  of,  247 

subperiosteal,  250 

varieties  of,  250 
Branchial  arches,  355 


Calcoglobulix,  231 

Calculus,  grinding  of  sections  of, 

473 
Cell  division,  337 
indirect,  338 

theorv,  336 

walls  of  plants,  238 
Celloidin,  blocking  of,  491 

cutting  of,  491 

method,  490 

summary  of,  492 

sections  of,  serial,  493 
staining  of,  492 

stock  solutions  of,  491 
Cement  corpuscles,  296 
Cementoblasts,  295 
Cementum,  188 

absorption  of,  200 

canaliculi  of,  194 

cement  corpuscles  of,  195 

distribution  of,  33 

embedded  fibers  of,  196 

function  of,  29,  189 

Haversian  canals  in,  188 

histogenesis  of,  189 

lacunae  of,  194 

lamella?  of,  190 

structural  elements  of,  190 
Chromic  acid  for  fixing,  498 
Cleaning  slides  and  cover-glasses, 

496 


506 


INDEX 


Cleaning  agents,  503 
Cleft  palate,  361 
Connective  tissues,  240 

chemical  relations  of  formed 

material  to  cytoplasm,  245 
mutations  of,  240 
relation     of,    to     mechanical 
conditions,  245 
Corrosive  subUmate,  497 
Creosote,  503 

Cutting  celloidin  sections,  491 
paraffin  sections,  487 


Decalcified  bone,  442 
Delafield's  hematoxylin,  500 
Dental  foUicles,  364 
ligament,  285 
papilla,  363 

pulp,  bloodvessels  of,  209 
cells  of,  arrangement  of,  209 

connective  tissue  of,  207 
definition  of,  201 
degeneration  of,  229 
from    unerupted    tooth   of    a 

sheep,  443 
function  of,  29 
sensory,  202 
vital,  201 
hard  formations  in,  235 
histogenesis  of,  203 
human,  normal,  444 
pathological,  445 
hyperemia  of,  219 
infarction  of,  224 
intercellular  substance  of,  209 
nerves  of,  214 
nodules  in,  229 
odontoblasts,  204 
pathology  of,  219 
preparation  of,  method  of,  493 
structural  elements  of,  203 
ridge,  362 
tissues,  28 

distribution  of,  30 
in  adaptation,  35 
Dentine,  caries  of,  157 
chemical  composition  of,  169 
dentinal  fibrils,  176 
distribution  of,  32 


Dentine,  function  of,  29,  167 
granular  layer  of,  178 
histogenesis  of,  167 
interglobular  spaces  in,  179 
lines  of  Schreger  in,  184 
matrix  of,  168 
secondary,  184 
sheath  of  Newman,  169 
tubules  of,  171 
diameter  of,  171 
direction  of,  in  crown,  171 
in  root,  174 
Dermal  scales,  22,  271 
Development,  beginning  of  calci- 
fication, 366 
chronology  of,  372 
of  dental  follicle,  364 
papilla,  363 
ridge,  362 
of  enamel  organ,  362 
of  permanent  molars,  first,  370 
second,  371 
third,  371 
of  tooth  germ,  362,  364 

for  permanent  teeth,  365 
Dissecting,  481 
Drawings,  427 
of  teeth,  425 

surfaces,  429 
of  typical  cavity  walls,  439 


Embedding,  483 
in  paraffin,  486 
Embryology,  335 

biological  considerations  fund^ 

mental  to,  335 
branchial  arches,  355 
chemical  ideas  related  to,  339 
early  stages  of,  340 
fertilization,  343 
formation  of  germ  layers,  347 
frontonasal  process,  357 
maturation,  340 
neural  canal,  352 
preparation  of  material,  495 
relation  of  cell  theory  to,  336 
segmentation,  holoblastic,  345 

mammalian,  348 

meroblastic,  348 


INDEX 


507 


Embryology,     separation  of  nose 
and   mouth    cavities,  360 
spermatogenesis,  340 
stomodium,  3.57 
,    transmission,  338 
Enamel,  action  of  acid  in  caries  of, 
150 
stages  in,  153 
areas    of    weakness    for    cavity 
margins,  incisors,  136 
marginal  ridges,  127 
simple    proximal    cavi- 
ties in  bicuspids  and 
molars,  137 
tips  of  cusps,  124 
atrophy  of,  structural  effects  of, 

160 
bands  of  Retzius,  60,  115 
chemical  composition  of,  39 
cleavage  of,  73 

cutting  of,  instruments  for,  76 
developmental  lines  in,  122 
differences    between    rods    and 
cementing  substance,  46 
from   other    calcified    tissues, 
38 
distribution  of,  30 
effect    of    caries    beginning    in 
natural  defect,   143 
on  smooth  surfaces,  145 
intensity  and  liability,  148 
secondary  or  backward  de- 
cay, 146 
on  structure  of,  143 
of  structure  on  cutting  of,  56 
etching  of,  48 
function  of,  28 
gnarled,  54 
growth  of  cap  of,  108 
lines  of  Schreger  in,  64 
mottled,  165 
occlusal  grooves  in,  115 
origin  of,  38,  362 
planing  of,  76 
refracting    index    of    rods    and 

cementing  substance,  51 
relation  of,  to  formation  tissue, 

41 
relative  solubility  of  rods   and 

cementing  substance,  46 
rods  of,  43 
short,  45 


Enamel,  straight,  53 
stratification  of,  60 
striation  of,  57 
structural  elements  of,  43 

form  of,  42 
walls,    structural    requirements 
of,  80 
bevel  of  cavosurf ace  angle, 

87 
classes  of  cavities,  87 
gingival     third     cavities, 

101 
incisor  pits,  105 
in  simple  occlusal  cavities, 

90 
steps   in   preparation   of, 

89 
support  of  marginal  rods, 
85 
of  worn  surfaces,  86 
supported  on  sound  den- 
tine, 80 
white  spots  in,  162 
Endoskeleton,  19 

relation  of  nervous  system  to,  22 
Epiblast,  347 

Epithelial  structure  in  peridental 
membrane,  307 
arrangement  of  cells  in, 

308 
distribution  of,  307 
Etching  and  mounting  ground  sec- 
tions, 429 
Exoskeleton,  19 

relation  of,  to  nervous  system,  22 


Farrant's  gum  glycerin,  502 
'  Fastening  teeth  to  grinding  disks, 
j      465 

;  Fertihzation,  343 
'  First  permanent  molars,  origin  of, 

370 
Fixative  for  paraffin  sections,  496 
Fixing,  481 

agents,  496 
Flemming's  solution,  496,  497 
Forces    influencing    bone    growth, 

393 
I  Formalin  for  fixing,  499 


508 


INDEX 


Formalin  for 

teeth,  424 
Frontonasal  process 


preserving   fluid  for   Growth  of  jaws,    relation    of    first 
molars,  398 
357  tissue  changes  in,  413 

of  mandible,  375 
of  membrane  bones,  261 
Gum  glycerin,  502 


GiNGivus,  gum  tissue  and,  447 

support  of,  291 
Gland  of  Serres,  310 
Glycerin  for  mounting,  502 
Granular  layer  of  Tomes,  178 
Grinding  of  crumbled  material,  473 
difficulties  in,  474 
disks,  457 

of  frail  material,  468 
in  hard  balsam,  469 
machine,  453 

grinding  of  sections  on,  463 
of  microscopic  sections,  453 

description  of  machine,  453- 

457 
fastening  teeth  to  grinding 

disks,  465 
frail  material,  468 
grinding  disks,  457 
lap  wheels  and  stones,  460 
management  of  balsam,  463 
measurement    of    sections, 

467 
point  finder,  460 
process  of  grinding,  462 
rapidity  of  grinding,  466 
removal  of  cover-glass  from 

disk,  471 
spatter  guard,  462 
spiders  and  dogs,  463 
waste  water,  462 
watering  stones,  461 
stones,  460 

clogging  of,  474 
of  tooth  sections,  425 
Ground  sections  of  bone,  441 
Growth  force,  392 
of  jaws,  390 

eruption  of  temporary  teeth, 

394 
growth  of  air  space  in  nose,  412 
importance  of  proximal  con- 
tact, 405 
influence  of  permanent  incisors 
and  cuspids,  403 


Hair,    teeth    and,    comparison    of 
origin  of,  25 
of  structure  of,  24 
Hardening,  481 
Hemalum,  500 

Hematoxylin,  Delafield's,  500 
eosin  and,  501 
Van  Gieson's,  501 
Histological  technique,  theory  of, 

478 
Holoblastic  segmentation,  345 
Homology,  22 

Hyperemia  of  dental  pulp,  219 
acute,  220 
chronic,  222 
Hypoblast,  347 


Indexing  and  filing,  485 
Infarction  of  dental  pulp,  223 
Inflammation  of  dental  pulp,  224 
Intercellular  substances,  236 

kinds  of,  238 

relation  of  cells  to,  237 
Isolated  enamel  rods,  435 


Jaws,  growth  of,  390 


Labelling,  485 

Laboratory,  manner  of  working  in, 

427 
Lap  wheels,  460 


INDEX 


509 


M 


Maceratiox,  480 
JMandible,  growth  of,  375 
Maturation,  3-40 
Membrana  eboris,  207 
Merkel's  cartilage,  367 
Methyl  green,  501 
Meyer's  fixative,  496 
Morris'  staining  dish,  489 
Mounting,  483 
Mouth  eavitv,  323 

epithelium  of,  323 

mucous  membrane  of,  323 

nerve  endings  in,  327 

submucosa  of,  325 

taste  buds,  331 

tongue,  327 
muscles  of,  328 
papillae  of,  329 
Mucous  membrane  of  mouth,  323 
Mailer's  fluid,  498 

formahn  and,  499 


Odontoblasts,  204 
Oil  of  bergamot,  503 

of  origanum,  503 
Osteoblasts,  297 
Osteoclasts,  299 
absorptions  by,  300 
in  burrowing  canals,  302 
Outline   drawings  of   ground   sec- 
tions, 432 
from    transverse    sections    of 
root,  440 


Par  AFFIX,  cutting  of,  487 
embedding  in,  486 
kinds  of,  487 

method,  summary  of,  489 
sections,  staining  of,  488 
Pathology  of  dental  pulp,  219 
Peridental  membrane,  279 
absorption  by,  300 
arrangement  of  fibers  of,  283 
bloodvessels  of,  313 


Peridental  membrane,  cellular  ele- 
ments of,  294 
cement  corpuscles,  295 
cementoblasts  in,  295 
changes  in,  with  age,  319 
definition  of,  279 
divisions  of,  280 
epithelial  structure  in,  306 
fibroblasts  in,  294 
fibrous  tissue  of,  283 
functions  of,  281 
longitudinal  sections  of,  450 
nerves  of,  318 
nomenclature  of,  280 
osteoblasts  of,  297 
osteoclasts  of,  299 
practical     considerations     of, 

321 
preparation  of  material,  494 
principal  fibers  of,  283 
relation   of   cementoblasts   to 

cure  of  pockets,  297 
structural  elements  of,  281 
transverse  alveolar,  449 
gingival,  447 
Periosteum,  262 

attached,  264,  267,  446 
complex,  270 
simple,  268 
classification  of,  262 
definition  of,  262 
functions  of,  262 
layers  of,  265 

macroscopic  appearances  of,  263 
preparation  of  material,  494 
relation    of    attachment    of,    to 

burrowing  pus,  264 
unattached,  complex,  265 
simple,  265 
Picric  acid,  498 
Placoid  scabs,  22,  28,  271 
Point  finder,  460 

Preparation  of  dental  pulp  mate- 
rial, 493 
of  embryological  material,  495 
of  grinding  material,  463 
of  peridental   membrane   mate- 
rial, 494 
of  periosteum  material,  494 
of  shellac  for  grinding  sections, 
472 
Preserving  tissues,  486 


510 


INDEX 


Rapidity  of  grinding,  466 

Reattachment  of  tissues  to  surface 
of  root,  297 

Relation  of  nucleus  to  cytoplasm, 
337 
of  section  to  crown,  424 
of  teeth  to  bone,  374 

to  development  of  face,  374 

Removal  of  cover-glass  from  grind- 
ing disk,  471 


S 


Safranin,  500 

Schreger's  lines  in  dentine,  184 

Secondary  dentine  and  cementum, 

study  of,  440 
Sectioning,  480,  483 

methods,  choice  of,  486 
Segmentation,  345 
holoblastic,  345 
mammalian,  348 
meroblastic,  348 
Serial  sections  with  celloidin,  493 
Sheaths  of  Newman,  169 
Silver  nitrate,  501 
injection,  502 
Slicing  mechanism,  475 
Spatter  guard,  462 
Spermatogenesis,  340 
Staining,  483 
agents,  500 
celloidin  sections,  492 
of  fresh  tissues,  479 
of  paraffin  sections,  488 
Stomodium,  357 

Structure  of  mandible  and  maxilla, 
377 
distribution     of     bone     in 
alveolar  process,  379 
in  mandible,  384 
in  maxilla,  390 


Structure  of  mandible  and  maxilla, 
influence  of  mechanical  condition 
in  evolution  of,  380 
Subperiosteal  bone,  250 

and    cementum,    comparative 
study  of,  442 


Taste  buds,  331 

Teasing,  479 

Teeth,  attachment  of,  271 

chisel,  36 

for  grinding  sections,  423 

grinding,  36 

relation  of,  to  bone,  27 
to  exoskeleton,  22 

temporary,  absorption   of  roots 
of,  302 
Tissue  changes   in  the   physiolog- 
ical movements  of  teeth,  413 
Tongue,  327 

muscles  of,  328 

papilla  of,  329 

taste  buds  of,  331 
Tonsils,  331 

lingual,  332 

palatine,  334 

pharyngeal,  334 
Tooth  germ,  364,  451,  452 

for  permanent  teeth,  365 
Transmission,  vehicle  of,  338 
Transverse    sections    of    roots    of 

teeth,  426 

W 

Washing,  485 
Watering  the  stones,  461 


Zenker's  fluid,  499 


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